fungus has a season of rest underground, and whether in the condition of resting spores, a sclerotioid mass or a number of mycelioid threads, the principal fact remains that the fungus lives through the winter in a state of rest. As to certain potatoes being able to resist the disease, I shall shortly be able to show that whilst certain breeds of potatoes entirely resist it in one place, they fall a ready prey to it in another.

Hence any experiments carried on in one place by one person, though valuable in themselves, must be inconclusive and imperfect.

The great question is, "How can the disease be evaded or destroyed?" and this can only be answered, if answered at all, by men who thoroughly know the fungus and its allies.

WORTHINGTON G. SMITH

The Denudation of Limestone Hills of Sarawak THERE is an agency in the denudation of the limestone rocks of Sarawak which I do not think has been noted, but which is very efficient locally in its operation.

The limestone in question is a dark-blue compact rock (probably the oldest stratified formation in this part of Borneo) full of fissures and joints, and forming hilly tracts in Sarawak proper and Samarahan. It is a not uncommon occurrence during periods of unusual drought for the jungle clothing these hills to take fire in some unascertained way, and for large tracts of the vegetation to be destroyed before the conflagration dies out or is extinguished by rains. Such an accident took place two years ago on the Jambusan hill, and a short time previously on Gunong Angus (whence the present name, "Burnt Hill"), and on Marajah, a large hill near Bidi; and I have been informed by natives that similar fires are known at the head of the Undup, where I have observed from a distance extensive masses of limestone. When such a fire takes place, not only may we take for granted that a great deal of surface-rock is more or less calcined, so as to be easily removable by the heavy tropical rains; but, there being no covering of soil to speak of, and the exterior rock having been merely bound together by a matted network of roots and creepers, large masses of rock-long loosened by weathering, or freshly detached by the expansion of air and water in the fissures-keep falling from the higher parts of the hill as their supports are burnt away; whilst groups of burning trees go crashing down the scarps, assisting the work of degradation by collision with the inequalities in their paths.

It is, however, subsequently to the fire that its most important effects become apparent. For the next year or two fresh dislodgments of rock will be continually taking place, particularly when, after the almost daily rains, the sun shines out, striking on the bared rock with rays of tropical fervour. Many years elapse before sufficient soil collects in the crevices of the rock to support vegetation; and until the whitened face of the hill is once more shrouded in jungle, it remains immediately exposed to steady sub-aerial denudation; so that, bearing in mind the immense rainfall, the abundance of fissures and joints in the stone, and its solubility, I am inclined to believe that the degra dation of these hills which goes on during the interval before they again become efficiently shielded with vegetation, is comparable to centuries of waste of the same rock under ordinary conditions.

Were the limestone hills of Sarawak more gently rounded and less scarped, their destruction through the agencies above described might not be noteworthy; but, owing to the frequency of lines of old sea-cliffs and mural precipices, nearly the whole of the detached rock passes at once to the bases of the hills, where it is again attacked by the rains, assisted now by running streams or standing water. Sarawak, July 1

A. HART EVERETT

An Appeal to our Provincial Scientific Societies Now that our provincial museums are yearly increasing in number, it appears desirable to draw the attention of the provincial scientific societies to their importance as the centres for the private collections illustrative of the local geology, natural history, and archæology which from time to time come into the market. We are entirely indebted to private energy for any British collections which we possess. How lamentable then is it that there is no public system for centralising them in our public museums, and thus saving them from dispersion by their passing into the hands of dealers or private collectors, or into the possession of foreign or metropolitan museums. Every year

witnesses such losses, which are regarded with complete indifference by our local representatives of Science. It is unaccountable that not one of our provincial Societies has as yet had the public spirit, energy, or foresight to see the importance of this work and of raising a fund for the purpose of ultimately securing such collections for the district.

It is a question of national scientific importance. The collections which are formed during the present century may be said to represent the " 'pick" of the country. By-and-by, when localities are worked out, and the rarity and value of specimens greatly increased, we may awaken to a sense of the mistake we have made in not devoting our energies less to paleontological literature, and more to the formation of complete and exhaustive local series and collections, and thus smoothing the path of, and providing interest for, the investigators of our fossil and recent flora and fauna.

Such is the lack of originality displayed in this country, and precedent is so blindly followed, that everywhere we find narrow scientific cliques, so-called "Societies," apparently formed merely for the sake of having social gatherings and by means of a local periodical facilitating the cheap publication of the papers of such

as contribute.

are

The energy thus expended is almost entirely thrown away. Indeed, so far as the journals of these "societies" concerned, these societies are mere hindrances to the progress of Science, for, did they not exist, the papers which appear in their obscure journals (or "napkins," in which the "talents of these societies lie hid) might be contributed to such as have a general circulation, and thus benefit the world at large. I would most earnestly impress on our scientific Societies the great importance of devoting their energies more to the formation and preservation of complete and exhaustive local collections. With such division of labour how much more accurate and rapid would be the progress of the sciences of Geology and Biology. S. G. P.

The Killing of Entomological Specimens

A NOTE in a recent number of NATURE, reminds me of some experiments I made about 15 years ago upon the action of the vapours of volatile liquids (hydrocarbons, chloroform, &c.) on insects, my object being to find an expeditious and painless method of killing entomological specimens. Several vapours produced insensibility from which the insects recovered more or less rapidly, but bisulphide of carbon vapour killed them effectually.

My method of applying it was to place a few layers of blotting paper, lint, or cotton wool, on the bottom of a wide-mouthed bottle, pill box, or other convenient place of execution; then to pour a few drops of the liquid upon this and confine the insect in the receptacle, which on account of the great density of the vapour need not be very accurately closed. The action of the vapour must be continued a few minutes after signs of life have disappeared, or the insect will recover.

The most obstinate of beetles succumb without a struggle, and the most delicate of moths or butterflies are uninjured, provided the liquid itself does not touch them. Butterflies may be killed after they are pinned out, by simply placing a little cotton wool soaked with the bisulphide in a box near to them. W. MATTIEU WILLIAMS

Lecture Experiments

THE result of convection in a liquid, tending to cause the upper part of the mass to be constantly at a higher temperature than the lower, may be well illustrated by the two following experiments :

Two large glass beakers are placed in front of a sheet of white paper, one of them filled with cold the other with boiling water. A boiling-tube filled with freshly prepared starch solution which has been coloured deep blue by gradual addition of aqueous solution of iodine, and has then been heated until the colour just disappears, is plunged into the beaker of cold water; the blue colour, caused to return by the cooling of the solution, will appear first at the bottom of the tube and then gradually creep upwards, showing that the lower part of the heated liquid first becomes sufficiently cooled to cause the return of the colour. In order to insure the disappearance of this colour by heat, an excess of iodine must be carefully avoided.

In the boiling water contained in the other beaker is immersed a boiling-tube filled with the blue liquid obtained by adding

caustic soda in excess to a solution of copper sulphate and tartaric acid, with which has been mixed a little grape sugar (a small quantity of "set" honey): the formation of yellow cuprous oxide commences at the surface of the liquid, and is seen gradually to extend to the lower parts, showing hat the upper parts first attain the temperature requisite to cause the reaction to occur which precipitates cuprous oxide.

These experiments are easy of execution, and by the above arrangement, or still better by being projected on the screen, may be rendered visible at a considerable distance. Queenwood College

FRANK CLOWES

Mr. Garrod's Theory of Nerve-Force THE thermo-electric theory of nerve-force propounded by Mr. Garrod (NATURE, vol. viii. p. 265) seems capable of extension. If a pole of metal, cased in a non-conducting sheath, were sunk in an artesian boring so as to reach from the level of constant temperature to the greatest depth attainable, how far would such pole fulfil the conditions of a sheathed nerve penetrating from the cool surface of an animal to the warmer interior? And with so little difference of temperature in so great a length, would its dynamic effect be at all appreciable?

A quarter of a mile of submarine cable let down the shaft of our Carnbrea mine might represent a sheathed nerve; and any existing nerve-force might there be tested. Abandoned mineshafts are the terrors of our Cornish moorlands. Is it within the power of Science to convert them into earth-nerves, say by lining their sides with non-conducting material, and then packing them tight with conductive slag or some kind of metallic refuse? And is it possible, even in theory, to make such earthnerves work some kind of earth-muscle? For ignorant me to speak of this subject is ultracrepidism (NATURE, vol. vii. p. 262). Yet it seems a fair extension of Mr. Garrod's ingenious theory. AUGUSTINE CHUDLEIGH Carnbrea, Cornwall

Genesis in Borneo

MR. CAMERON's paper read at the Society of Biblical Archæology, testifies to the early diffusion of Semitic traditions by the agency, it may be inferred, of Moslem converts.

The same traditional coincidences recorded of Borneo are found in New Zealand and elsewhere, and would naturally accompany the diffusion of Malayan dialects throughout Polynesia, an influence the duration of which may be counted by A. HALL centuries.

Dec. II

Indian Snakes

IN a small treatise on Indian snakes by Dr. Nicholson, R. A., the author states his belief that cobras will not feed in captivity unless forced to, starving themselves voluntarily to death. He thinks, also, that jugglers in this country either "feed their cobras with liquid nourishment, or else let them loose when their lives are in danger," recapturing them at a future time.

To test the correctness of this, I questioned a snake-charmer a few days ago, and he informed me that he fed his cobra every week with frogs. His snake had then been recently fed, so he was told to bring it to the bungalow again in a few days. A frog (R. tigrina) was procured, and placed in the small basket in which the cobra was kept. The latter seized it at once; but as I was anxious to see the whole process, which could not be done whilst the snake was coiled up in the basket, I requested the man to place the frog on the ground. As it struggled away (the hind limbs of the poor reptile had been broken) the cobra followed it eagerly, and again and again seized it. The want of fangs, and the size of the frog, which in its inflated state exceeded considerably the circumference of its enemy, rendered these attempts ineffectual; so a smaller frog was caught, and placed with the cobra in the basket. This was swallowed in a short time, the snake pushing its victim against its coils, and working down the hind limbs by a lateral motion of the lower jaw, very similar to that of a cow chewing the cud.

The large frog was now placed in the basket, and the cover put on, and in about half an hour had followed its companion. The cobra's appetite was now appeased, for after seizing a third frog it let it go, on its croaking a remonstrance.

A laughable incident occurred whilst the snake was following the frog over the gravel path. A performing monkey belonging to the juggler, in a spirit of mischief, or perhaps fearing that its master's

property was escaping, stepped gravely after the snake and laid hold of it by the tail. As a natural consequence, round came the cobra and menaced the monkey, which, retreating with sundry grimaces, took refuge with the juggler, in great alarm at the turn events had taken.

This cobra is a small one, and as it is one of those very pale, almost cream-coloured varieties, that finds no mention in Günther's able work, I am anxious to examine it thoroughly. The owner, however, affirms that he has to draw its fangs about once a month, and as he is most cautious in handling the reptile, it is probable that the fang matrix has not been destroyed, and examination will be safest just after the operation of extracting the fangs. E H. PRINGLE

Mangalore, Sept. 12

IN

CLASSIFICATION OF CLOUDS*

N an essay on the "Modifications of Clouds, read to the Askesian Society in 1802, Howard first proposed his classification of clouds, which has since been the generally received authority on the subject. His system has thus stood its ground for more than half a century, in spite of its defects and of the misconstruction not unfrequently put on the two terms, "stratus" and "nimbus " since the publication of Kaemtz's Meteorology. These misapprehensions and the obscurity and confusion arising from them are pointed out by Prof. Poey, but the errors have not been followed so generally as is asserted, at least by British meteorologists. In a series of papers issued at intervals during the past eleven years, Prof. Poey has endeavoured to develop a new classification of clouds, of which the volume before us is the result.

The following is Poey's classification compared with that of Howard :

In forming his system, Prof. Poey first strikes out the stratus" as being from Howard's own definition not a true cloud, but only mist ;" the "cumulo-stratus" as not differing really from the cumulus; and the "nimbus" as being not a single cloud, but rather a system of clouds. He retains the word "stratus" as part-descriptive of the "cirro-stratus," but in this case it is exclusively restricted to those instances where the cirrus arranges itself in a stratified form, and is not applied when the arrangement is an extended sheet or continuous layer of considerable thickness totally impervious to the sun's rays. To this latter condition, the new term "pallium" is applied.

In his classification Poey arranges the clouds in the order in which they severally appear, from the cirrus, the most elevated, its height being from 30,000 to 50,000 feet, to the fracto-cumulus, the lowest of all; and groups them into three divisions according as they are composed of ice-crystals, snowy particles, or vesicular vapour.

But the most fundamental change which he has introduced into the system is the pallium or sheet-cloud, in its two distinct forms of pallio-cirrus, and pallio cumulus, according as it is formed from the cirrus or the cumulus. The pallium is the greyish, or ash-coloured cloud which overspreads the whole sky, and from which rain falls continually for hours or days together. On the approach of rain the pallio-cirrus is formed by the rapid increase and thickening of the cirrus downwards from the enormous

* "Nouvelle Classification des Nuages suivie d'Instructions pour servir à l'Observation des Nuages et des Courants Atmosphériques." Par André Pocy, Havane. (Extrait des Annales hydrographiques, 1872.) Paris, 1873. (17 Planches).

accessions of moisture that take place, by which this high ice-cold region of the atmosphere over a great extent and thickness, is brought to the point of saturation and condensation. Underneath this leaden-hued mass of cloud which uniformly covers the sky, but separated from it by a clear space, is extended the dense cloud covering of the pallio-cumulus, which is formed by the watery vapour of the atmosphere reduced to the points of condensation and precipitation. This is the true raincloud, and it is fed and increased by the rapid drifting in from below of torn masses of cumulus constituting the fracto-cumulus or wind-cloud. The fracto-cumulus may be of all sizes, has no determinate shape, is the lowest and swiftest moving of the clouds, and is whitish, greyish, or slate-coloured, as may be determined by the hygrometric condition of the air. On the return of fine weather accessions of vapour by the fracto-cumulus slacken and then cease, the pallio-cumulus diminishes in thickness and gradually clears away, showing through its intervals the pallio-cirrus above it, which in its turn is broken up, revealing still higher up the delicate tracery of the cirrus. The pallio-cirrus is negatively electrical, whilst the palliocumulus is positively electrical, the clear stratum between being neutral; and between these oppositely electrified strata, discharges frequently take place in thunderstorms. The merits of Prof. Poey's work are very considerable, whether they be regarded as expository of Howard, or as a contribution to this difficult branch of meteorology; and it is just those meteorologists who have paid particular attention to the observation of the clouds who will be readiest to recognise its merits. It must, however, be conceded that, as a descriptive classification of clouds, as well as explanatory of the phenomena they present, Prof. Poey's work leaves the subject in a state still too incomplete to warrant us in recommending his system for general introduction. It is a step in the right direction, and will materially contribute to place this vitally important department of atmospheric physics on a satisfactory footing. Toward this end, what is now urgently wanted is an extensive collection of the data of cloud-phenomena in all countries, particularly of those clouds interesting in themselves or from their known relations to weather changes. We have more than enough of unmistakeably pure typical forms scattered through the pages of weather-literature, but such do not greatly assist us, in describing and classifying many of the forms of clouds which occur. Hence what is required is faithfully accurate delineations of these forms in their different aspects, and systematic inquiries set on foot into the relations of the forms of clouds to the mode of their formation, to the states of the aqueous vapour which compose them, and to the varying elasticity, temperature, and electricity of the atmosphere.

In connection with this part of the subject, Prof. Poey investigated in 1862-64, by means of the thermo-electric pile, the temperature of different parts of the sky under different conditions, and of the clouds which passed across it. Among other highly interesting results, he has shown that the cumulus, properly so called, and the cumulostratus of summer are the clouds of highest temperature; then follows the fracto-cumulus, except when it comes after the rain which accompanies a thunderstorm, in which case it is of a whitish colour, very rapid in its motion, much torn at the edges, and partakes of the low temperature prevailing on such occasions. The cirrocumulus is colder than the cumulus and the cirrus the coldest of all the clouds. These are very suggestive results. We are convinced that the key to the position in meteorology is a better knowledge of the vapour of the atmosphere in its various states and changes; and the science will not make the advances it is destined to make till meteorologists generally recognise the necessity of equipping their first-class observatories with the requisite appliances for carrying on those physical researches which are intimately allied to meteorology.

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FERTILISATION OF FLOWERS BY INSECTS

V.

More conspicuous flowers adapted to cross-fertilisation, and less conspicuous ones adapted to self-fertilisation, occurring in different species of the same genus.

WHA

HAT has been described in the two last articles as occurring in varieties of the same species (using the term species" in its widest sense) we propose now to investigate as existing likewise in species of the same genus.

Malva sylvestris and rotundifolia

are two closely allied, but, as acknowledged by all botanists, undoubtedly good and distinct species, differing in their flowers in a manner similar to the two varieties of Lysimachia vulgaris and the other species previously considered. In both these species of Malva an oval mass of anthers in the first place occupies the middle of the flower, enclosing the stigmatic branches as yet undeveloped and lying close together (Fig. 23). At a later period the stigmatic branches, growing out of and overtopping the mass of anthers, spread and bend outwards and downwards so as to occupy nearly the same place as was before occupied by the anthers (Figs. 24, 25). Insects, therefore, seeking for the honey which is secreted and contained in five cavities between the lowest parts of the petals (», Fig. 23) and covered by a fringe of hairs (pr), carry away on their hairy bodies the large prickly pollengrains from younger flowers, leaving many of them on the stigmatic papillæ of the branches of the style of older flowers, which they can scarcely avoid grazing in seeking for the honey. Hence, in both species, whenever insects frequently visit these flowers, cross-fertilisation in the manner described is largely effected, whereas self-fertilisation can scarcely take place, neither spontaneously nor by means of insects, nearly all the pollen-grains having been removed before the unfolding of the stigmatic branches. Since, however, Malva sylvestris and rotundifolia grow for the most part in the same locality, and flower during several months at the same time, insects flying about and seeking for honey are much more likely to find out and visit the highly conspicuous flowers of M. sylvestris than the far less conspicuous ones of M. rotundifolia, the former, when fully opened, presenting bright rose-coloured bells of from 40 to 50 mm. diameter, the latter, on the contrary, light rosecoloured bells of only from 20 to 25 mm.

Direct observation, indeed, fully confirms this supposition, the flowers of M. sylvestris being always found in sunny weather visited by a variety of insects, whereas those of M. rotundifolia, especially when growing intermixed with M. sylvestris, are commonly overlooked by them all. Thus, during the six last summers, I have observed on the flowers of M. sylvestris and collected more than 50 species of insects, many of them very frequently (2 Lepidoptera, 3 Diptera, 5 Coleoptera, 40 Apidæ, some Ichneumonidae); while in the same space of time I found on the flowers of M. rotundifolia but 5 species (4 Apidæ, 1 Hemipter), and those only in single or a few cases.

It is evident from these facts, that wherever our two species of Malva grow together in the same locality, M. rotundifolia would be rapidly extinguished, unless it were enabled to produce seed by self-fertilisation; M. sylvestris, on the other hand, is so commonly visited and cross-fertilised by insects that self-fertilisation, if it were possible, would never be effected, or only exceptionally. Accordingly natural selection must have preserved and accumulated those slight individual variations of M. rotundifolia, which afford facility for self-fertilisation, whereas in M. sylvestris the possibility of self-fertilisation being quite useless, might be lost, and, indeed, has been, completely or nearly lost. Thus in the flowers of M. sylvestris, when precluded from the visits of insects by covering them with

a net, the anthers remain filled with pollen-grains, and never, or only exceptionally, come spontaneously into contact with the stigmatic branches, the free ends of their filaments at a later period bending downwards, and the branches of the styles remaining considerably above them (Fig. 24). Conversely in the flowers of M. rotundifolia, when the visits of insects are prevented, the anthers, filled with pollen-grains, remain in so high a position, and the stigmatic branches bend so far downwards as to come abundantly into contact with the pollen-grains, selffertilisation being thus inevitable (Fig. 26).

Epilobium angustifolium and parviflorum differ most strikingly in a similar manner. The flower of E. angustifolium, being of larger size, brighter colour, grouped in long splendid clusters, and exciting attention at a great distance, are so largely visited and cross-fertilised by insects* as never to have need of self-fertilisation, which has actually become impossible; the four stigmatic branches unfolding so long after the maturity of the eight anthers, and so far overtopping them, as to be completely shut out from the pollen of the same flower. The flowers of E. parviflorum, on the other hand, being of smaller size, lighter colour, and single, are so inconspicuous that insects but very rarely visit them. Accordingly, its four upper anthers so closely surround the four-lobed stigma, which is mature at the same time, as to cover it largely with their pollen, whilst the pollengrains of the four lower anthers lying on the way to the honey, cannot reach the stigma of the same or of another flower unless transferred by insects.

Polygonum

With re

Among the many species of the genus Polygonum which grow in our country there are two, P. Fagopyrum and Bistorta, most distinguished by their attractiveness for insects, which is due not only to the size and colour of the single flowers and to their collection into handsome spikes, but also, and even more perhaps, to their abundance of honey secreted by eight globular nectaries at the base of the filaments (", Figs. 26, 27). ference also to the frequent visits paid them by insects, † these two species have been adapted to inevitable cross-fertilisation by their visitors, self-fertilisation having at the same time become difficult or almost impossible. The manner in which this advantage has been attained being very different in the two species, it is evident that in this case the adaptation to cross-fertilisation by the visits of insects cannot have been inherited from the common parents of the genus, but must have been acquired by the single species during their evolution.

P. Fagopyrum has acquired, as shown in Figs. 26 and 27, the same kind of dimorphism which has been so fully explained by Darwin in Primula and Linum. § In both of the two kinds of flowers (which occur only on different plants) there are three styles and eight stamens, three of the stamens closely surrounding the styles and opening outwards, the five others inserted more outwards, alternating with the leaves of the perianth and opening inwards. An insect, therefore, visiting a flower for honey and pushing its head or proboscis between the inner and outer stamens into the base of the flower, cannot avoid being charged with pollen, especially in those parts of

* On the flowers of Epilobium angustifolium I have hitherto observed 26 species of insects, 14 of them belonging to the family of bees, many of them very frequently; on those of E. parviflorum I found only once Meligethes, and once a butterfly (Pieris rapa L.) repeatedly sucking the honey of its flowers.

On the flowers of P. Fagopyrum I have observed 41 species of insects, among them 21 Diptera and 12 Apida; on the flowers of P. Bistorta 18 species of insects, among them 9 Diptera and 3 Apida; many of the visitors of each species very frequently.

On the two forms or dimorphic condition in the species of Primula and their remarkable sexual relations (Proc. of the Linn. Soc. vi. (1862); Bot. PP. 77-79.

On the existence of two forms and their reciprocal sexual relation in several species of the genus Linum, Ibid. 1863, pp. 69-83.

its body which, whilst it is sucking the honey, are pressed against the anthers. Now, the place occupied in one of the two kinds of flowers by the anthers, is occupied in the other kind by the stigmas, the same parts of the body of the insect which in the long-styled form were pressed against the anthers, come into contact in the short-styled with the stigmas, and conversely. Thus it is inevitable that insects effect chiefly what is called legitimate fertilisation, i.e. transmission of the pollen of the long-styled flowers to the stigmas of the short-styled, and of the pollen of the short-styled to the stigmas of the long-styled form. Fertilisation by pollen of the same form, however, and even of the same flower, is not impossible, and in the short-styled flowers even spontaneous self-fertilisation may happen, by pollen-grains failing down from the anthers upon the stigmas.

The same advantage which P. Fagopyrum has attained by dimorphism (Darwin) or heterostyly (Hildebrand), has been gained in the flowers of P. Bistorta by protandrous dichogamy, i.e. by the anthers so far preceding in their development the stigmas that in the first period of the flower (Fig. 28) only mature anthers, at a later period (Fig. 29) only mature stigmas are present, the anthers having then commonly fallen off. It is readily seen that such flowers also, when perseveringly visited by insects, are always inevitably intercrossed, no other mode of the transmission of pollen being possible than from younger flowers to the stigmas of older ones. It is only when the visits of insects are completely wanting during the first period and the anthers remain clothed with pollen while the stigmas attain their maturity, that self-fertilisation by insects or even spontaneous self-fertilisation is possible.

The least attractiveness for insects, on the contrary, among all native species of Polygonum is possessed by P. aviculare, its flowers (Figs. 30 and 31) being of small size, of greenish and white or reddish colour, standing singly on procumbent plants and offering only a small quantity of pollen to insects, but, as far as I have been able to see, no honey. No wonder that insects are induced only in very rare cases to visit and fertilise them," and that, in compensation for the loss of cross-fertilisation, these little flowers regularly experience spontaneous self-fertilisation, the three inner anthers lying so close to the stigmas that their pollen-grains inevitably come into contact with them (Figs. 30 and 31).

Of the many other native species of Polygonum, which are all intermediate, as to their attractiveness for insects, between those now described, I will only remark briefly upon P. Persicaria, which is of more especial interest because of its flowers presenting great differences of structure. In this species, instead of eight nectaries there are only five developed, and these secrete a much smaller quantity of honey than those of P. Fagopyrum and Bistorta. Its spikes of flower, moreover, being less conspicuous than in those species, the visits of insects are somewhat rare, even in sunny weather, although far more frequent than in P. aviculare.+ Fertilisation by insects, consequently, is by no means secured. Corresponding to this uncertain agency of insects the sexual organs of the flower are in a remarkably fluctuating condition, undecided, as it were, between adaptation to crossfertilisation by the visits of insects, and to self-fertilisation. Thus, of the eight stamens, sometimes only the five outer ones are developed, the three others being reduced to rudimentary filaments; and this condition is apparently the most favourable to cross-fertilisation, as any honey-seeking insect must touch the anthers in every flower with one side of its proboscis, the stigma with the opposite side, to which it thus cannot fail to transfer pollen-grains

*After having repeatedly in vain watched P. aviculare in very hot sunny noons of the month of August 1871, I succeeded in observing some small Syrphide (Ascia podagrica F., Syritta fipiens L., and Melithreptus menthastri L.) visiting its flowers.

+ I have observed in the flowers of P. Persicaria altogether 11 species of insects, among them 7 Diptera, and these as the most frequent visitors.