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RECENT OBSERVATIONS ON THE FERTILISATION OF PLANTS.
BY ALFRED W. BENNETT, M.A., B.Sc., F.L.S.
T is proposed in the following article to give an account of some of the most recent observations on the subject of the contrivances by which the Fertilisation of Flowers is effected ; a subject the details of which are so numerous and varied that the field of observation open, not only to the scientific botanist, but even to the ordinary observer, seems almost boundless. So much' has now been written on this subject that everyone who has followed it to any extent is aware that the greater number of flowers are cross-fertilised—though to this rule there are exceptions to which we shall allude presently—and that the mode in which this cross-fertilisation is usually effected is by the agency of insects. There are, however, a considerable number of flowers which are fertilised without the assistance of insects by means of the wind; and as these present, as a class, peculiar features of their own, we may spend a little time in the first place in considering them.
A very good and familiar example of flowers fertilised by the agency of the wind is furnished by the common hazel, which flowers from January till the early part of March, even when the weather is very cold, and when there are scarcely any insects on the wing. The flowers of the hazel are of two kinds, male and female. The male flowers constitute the familiar catkins, which drop off and disappear before the leaves make their appearance. The catkins are generally in bunches of from two to four, every catkin containing on an average perhaps from 100 to 120 flowers. Each of these male flowers consists of a simple scale-like bract enclosing from eight to twelve anthers, each of which discharges, when ripe, a cloud of innumerable pollen-grains; so that the number of these grains in any single catkin must be prodigious. The female flowers are found on
VOL. KIL—NO. XLIX. z
the same branches as the catkins, and are also in clusters of from two to six or eight (the future nuts), and are of equally simple structure with the male flowers, being formed of a single pistil enclosed in bracts, the ovary surmounted by from three to five stigmas, the bright crimson threads by which these female flowers are recognised. If one of these crimson threads is placed under an ordinary pocket lens, it will generally be found to have on its surface several apparently minute particles of dust, which, on further examination, are found to be pollengrains which have been blown from the male flowers. Each individual pollen-grain has the power of emitting a “ pollentube,” which penetrates the stigma, reaches the ovary, and by the fertilisation of the ovule induces the formation of the embryo, and thus the development of the ovule into the fertile nut. Since the only means by which the pollen can be conveyed from the male to the female flower is the agency of the wind, and it is only quite by chance that any of the grains can reach their destination, the reason is obvious of the enormous amount of pollen with which the catkins of the hazel are furnished. In some plants, the fertilisation of which is effected in the same manner, the quantity of pollen is still greater, and this is especially the case in the Coniferae or fir-tribe. If a. yewtree is struck with a stick or agitated by the wind at the time when the pollen is being discharged, it will rise in the form of a. dense smoke, giving the impression of a burning bush; and American travellers have described how the water of some of their lakes near the shore is coVered at certain seasons by a. thick stratum of a. sulphur-like substance, the pollen blown from the neighbouring pine-woods. Whether the female flowers of the hazel are fertilised from the catkins on the same or on a. different bush is a point still in dispute.
Another instance in which there is little doubt that fertilisation is accomplished by the agency of the wind, though botanists are not quite unanimous on this point, is that. of our common cereal crops, and especially of wheat. Important in the highest degree from a mere mercantile point of view as is any question connected with the production of our corn crops, it is only very recently that any reliable observations have been made on the mode in which the flowers of wheat are fertilised; but these have led to some very curious results?“ When a field of wheat is in flower, that is, in ordinary seasons, in the early part of June, each ear will be found to be furnished with a great number of purplish anthers hanging at the ends of filaments of extraordinary delicacy, or rather of empty anther-sacs from which every grain of pollen has been discharged. These anthers
" See “Gardener’s Chronicle,” March 15 and 22, and May 24, 1873.
appear, when they have arrived at maturity, to break suddenly out of the opening bud, the filament elongating in a. moment to several times its original length, the anther bursting at the‘ same time, when the slightness of its attachment to the filament causes the least breath of wind to sweep the whole of the light dusty pollen out of its case, some of which must necessarily reach the neighbouring stigmas in the same ear, provided there is not enough wind to blow it completely away. In rye and oats this extraordinarily rapid lengthening of the filaments is even more conspicuous than in wheat. Hence the importance attached by farmers to comparatively calm sunny weather at the critical period when the corn is in flower.
These two examples furnish good illustrations of the structure which prevails in those flowers that are fertilised by the wind. They are generally of very simple structure, and rarely brightly coloured, since bright colours would be of no advantage to them. The quantity of pollen is usually very large, and the structure of the male flowers such that it is dispersed by the wind with the greatest facility, this being brought about by the slender “versatile” filaments of the wheat and by the lightly hanging catkins of the hazel, the willow, and other early-flowerin g shrubs, which appear before the leaves, and hence at a. period when there is no obstruction to the free dissemination of the pollen. ‘
In the majority of flowers, however, the structure of the pollen, or the arrangement relatively to one another of the pistil and stamens, is such that fertilisation could not be effected by the wind alone. Sometimes the pollen-grains themselves are too large and heavy to be thus conveyed, or they are united together by fine threads or even into dense masses; or the position of the stigmatic portion of the pistil is evidently not adapted for the pollen to reach it in this way; and Nature then employs as the agent in fertilisation the services of insects or of other small animals. This opportunity is afforded by the visits of insects to the flowers in search of the honey or nectar which forms an important portion of the food of many classes. The mode of attraction to the flowers which serve them for food is mainly two-fold, scent and colour; in other words, those properties which chiefly render flowers attractive to our own senses. The honey or other sweet juice is generally stored in small glands or receptacles, which together form the “ nectary,” the position of which is extremely variable; the deep pits at the base of the corolla. in the crown imperial, the small scroll-like petals of the hellebore, the bottom of the spur in orchises and the larkspur, the prolongations of two of the stamens which project into the spur of the violet and pansy, very frequently minute glands at the base of the stamens or pistil, &c. Nature is always economical of her resources; and accordingly we do not generally find that strong scent and brilliant variegation of colour are bestowed on the same flower. Those which are most prized for the power or delicacy of their scent have, as a rule, flOWers either inconspicuous, or, if large and conspicuous, of uniform unvariegated colour; as, for instance, the mignonette, Daphne, primrose, sweet violet, lily of the valley, rose, evening primrose, pink (in its primitive white state), honey-suckle, lime-tree, and many others; while the most brilliantly-variegated flowers are comparatively or quite scentless, as the fritillary, Pelargonium, larger and smaller Convolvulus, Tropaeolum, Mimulus, Ranunculus, pansy, &c. In scented flowers the scent proceeds from the nectar itself, and is therefore a sufficient guide for the insects in search of it. One of the largest of scented flowers, the evening primrose, blossoming only in the night, is fecundated by night-flying moths, which probably require the large sulphur-yellow flowers, as well as the scent, to guide them from a distance in the dim light. A distinction may also be drawn in general terms between the mode of fertilisation of large conspicuous and of smaller variegated flowers ; the agents in the former case being generally large insects, butterflies, moths, beetles, or bees; in the latter very much smaller ones. If a watch is kept on very large flowers, such as the single hollyhock, single peony, “ Convolvulus major ” of the gardens, the large white wild Convolvulus, Fuchsia, &c., it will be seen that their visitors mostly consist of large beetles, hive or bumble-bees, or butterflies, while the small flowers are overrun with small flies or other minute insects to whom the variegation serves as a guide, the streaks or rows of colouring invariably pointing to the nectary or receptacle of honey. American naturalists state that many of the largest and most gorgeous flowers of the western continent, such as the Bignonias or trumpet-flowers, are fecundated by humming-birds. A very good illustration of the difl'erent contrivances exhibited by two closely-allied plants—one scented and fertilised by bees, the other scentless and variegated and fertilised by very minute insects—is afforded by the sweet violet and the pansy.*
If attention is paid to the arrangement and position of the stigmas and stamens at the time when insect-s are seeking the flowers for the sake of the honey, it will be seen that the anthers are almost always at this time discharging their pollen, and that it is impossible for the insect to find its way to the nectary, or to insert its proboscis into it, without brushing against one or more of the anthers, and carrying away with it a portion of the pollen. Either in its retreat from the flower or in entering the
' See “ Nature,” vol. viii. p. 49, May 15, 1873.