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greens and other plants, as the cabbage and meadowgrass, which maintain their verdure at such times? However, since there cannot in anything well be effect without cause, let us seek what ground they afford us of offering an explanation of such a difference. In doing this we shall not have occasion to look very far before we shall find, in the examination of the leaves of the first, and I might add, main class of plants, i.e. the evergreens, something tangible to theorize upon. For regarding the leaves of plants — as is generally conceded - as the lungs, through which the functions of respiration and exhalation take place, we shall not be long, I think, before arriving at something like a definite conclusion on the matter. In the leaves of evergreens, or as I will now call them for sake of convenience, cold-adapted and heatadapted plants, there has been made a provision against too rapid exhalation and inspiration, through which they become naturally adapted for both conditions—either of heat or cold. Hence, bearing in mind what has just been stated in reference to the relation which the chlorophyl sustains to light, warmth, and sap, and also of how this latter stands affected to cold, there will be perceived the feasibility of the conclusions I am here seeking to establish; and to show that, as in cold-adapted plants so in tropical, the same conditions are necessary to meet either extreme, I need only mention the fact that were it not so, they would not be fitted to protect the sap against the too detrimental effects of extreme cold on the one hand, or of heat
on the other, because the sap in plants, as in animals,
is the life; and consequently connected with the
presence or absence of which, in part or whole, will
be the corresponding issues-life or death, of that part or whole. The nature of the protection of the leaves of these plants, as contrasted with those which are so readily and lengthily kept denuded, will be perceived to consist in their clothing membrane or epidermis being formed, in point of texture, much thicker and stouter. It of course is this increased thickness and hardened texture of their epidermis which give the leathery or woody leaves of evergreens their peculiar appearance and consistence, enabling them to withstand external influences of heat and cold so long. So in respect of young shoots; these, when they emerge from the bud, are covered by a delicate epidermis, by which they are enabled to retain their green colour and succulent condition for some time. Seeing then that the epidermis is so specially designed to prevent a too rapid evapora. tion of fluid matters from the tissues beneath, so must it follow that, according to the nature of the epidermis will its adaptability or non-adaptability be in protecting the sap; just in proportion to which being protected, will the greenness and vitality, or otherwise, of the leaves be made manifest. It might be asked, what can be advanced to
account for plants in whose foliage the same conditions are wanting, as in evergreens, cabbage, and some of the grasses? To explain this, I regard it as of primary importance that the same facts be still borne in mind with which I started, coupling them with considerations such as these:—(1) The relative difference between the temperature of the earth and that of the atmosphere; (2) the close connection existing, as a rule, in all such instances (for be it here remembered these plants are mostly acauliferous) between underground stem and leaves of these plants. For such cases, where we have the sap laid up in a part so protected as an underground stem, from which the leaves do so immediately spring, can scarcely fail but to maintain a sufficiently vigorous vitality in the leaves themselves. Upon an almost similar principle is it that I account for the reason of petiolate or stalked leaves falling more readily than those that are sessile—the two cases being almost parallel. Of course, viewing, as must be done for the sake of analogy, that the woody stem to which such a leaf is attached, and its petiole, as each severally corresponding to the protecting nature of the earth on the one hand, and the exposed stalk to the subterranean and consequently protected stem of the other, it will be perceived how it stands to sense why such a leaf should sooner defoliate than the other, where no such inter-appendage as a leaf-stalk divides the two, because the petiole cannot be regarded as a whit hardier than the leaf itself, since both, in point of system, are similarly constituted. I do not intend that what I have just said should apply to others than perennials, that is, plants that live for many years, and not of those which by nature are limited to a fixed term of one or two years. But even connected with the fruiting and defoliation of these, and indeed all of them, there is something to be learnt. For is it not notable that defoliation in no case precedes fruiting, but succeeds it? And why? I think the answer is not far to seek, even to one very moderately qualified in the rudiments of botanical science. Is not the period of fruiting one in which the secretions are more attracted, and perhaps altogether in the case of annuals and biennials, to the fruit?—hence most probably their death. Whereas in the case of others this process of developing and maturing their fruits would appear to be only so far exhaustive of the energies of the plant as to partly deprive the leaves of their vitality; and thus aiding their defoliation,-a theory, by the way, which seems to be well substantiated in the case of the holly, which is known not to shed its leaves till early in the spring, directly after the berries have ripened. Now there is, I know, a very common, though notwithstanding, erroneous impression among some people that the holly never sheds it leaves at all; but this no doubt arises from the fact of its never being seen perfectly denuded thereof; for no sooner do the old ones drop than fresh ones appear;-hence in such a sense it is evergreen. What makes me attach still more importance to the effects which flowering and fruiting have in partly aiding defoliation, is due to the case of a tree, possessed by a friend of mine, coming under my notice only a short time ago. This tree, which he has had now for close upon four years, has never parted with a leaf during the whole of the time, yet looks as healthy and vigorous as ever it did. Now I cannot account for this on any other ground than that of its never having flowered in the time. Certainly the plant is not elbow-jointed where the leaves join the stem; still I cannot see how this can well be accepted as a main cause. Then there is the question of articulation to be considered as effecting defoliation. This ought not, as some seem to do, to be regarded as fully causative. It may, and indeed no doubt does, after the sap becomes less active
through the chilling influence of cold, or other
causes, induce the leaves to fall more rapidly; still even this would be very much dependent on the nature of the wood of the plant. To mention a case as illustrative of this point, I would allude to the beech—a tree, among others, that retains its leaves in a decayed condition throughout the winter, even to the shooting of the buds. Now I am so bold as to think that even their points of attachment would become very much strengthened could but the vitality of the leaves be maintained beyond a year or so under favourable climatical influences. Then as to trees, there is another thing which ought not to be lost sight of in the consideration of this subject. I refer to their height, and the increased coldness to which the leaves are as a consequence subjected, all which must have, and especially in cases where the leaves are tender and gifted with “free lungs,” a very great influence in hastening and prolonging their nudity. In proof of this witness the effects of a mild winter; for scarcely will vegetation have had time to replenish itself from the impoverishing effects of fruiting before the activity of the sap will be again manifesting itself through the appearance of buds, showing how cold keeps in abeyance the sap, and chiefly through that causes defoliation and length. ened nudity, and also where in some instances it has reached the plant in its most sheltered partsthe roots—it has killed it altogether. Perennials, or those in which such phenomena as defoliation, &c., take place, are for the most part ligneous or subligneous in their structure; and depending upon this is their power of endurance and resistance. Hence I take it that the leaves, flowers, and fruits, together with their petioles and peduncles, being chiefly made up of a softer tissue, are sooner perishable, and as a consequence fall away. Especially so does this appear in a measure
partly explanatory of such phenomena, when we don’t find, as a rule, such appearance occurring amongst plants which are herbaceous, or, in other words, those in which the parenchymatous system predominates throughout; as in these we find all equally perishing throughout, without any such separation of parts. Apparently confirmatory of this we will take the effects of a keen prolonged frost upon newly-formed wood or young branches; the result being that of its becoming nipped, giving rise to what here might also be equally and feasibly claimed as the “Phenomenon of Delignization.”
As to what appropriate warmth, soil, and moisture will do in keeping up the vitality and foliage of plants generally, we have well exemplified at home in our nurseries, where a case of complete plantnudity would rarely or ever be witnessed. We have but to study the geographical distribution of plants in order to gain a correct knowledge of how far soil and climate go towards influencing vegetable growth. If, for instance, we were to go to Egypt, we should there find plants going through the phases of their existence in one half the time they do here. But, on the other hand, we should find, as we proceeded from warmer regions towards the poles, that, as the light and heat diminished, the vegetation is checked in the same proportion; proving that it is only where the sun rises highest in the ever-cloudless heavens that vegetation flourishes in the greatest luxuriance, and assumes its most majestic form.
I hope that this imperfect paper may be the means towards eliminating something further on this most interesting subject through the pages of SCIENCE-GossIP.
Sheffield. JoHN HARRISON.
TH: are few living authors who have intro
duced more students to the various branches of natural science than the Rev. J. G. Wood. As an author he has the happy knack of immediately striking a friendship with his readers, unless, indeed, they are more captious than usual. The entire field of zoology has been roamed over by him, and there is hardly one of its corners he has not explored. The wonder is, not that amidst so many books written by one man, there should be some errors in fact and errors in judgment, but that there are so few. If there is a tendency sometimes to dismiss those leading speculations which are agitating the minds of the best and most philosophical naturalists in the world, somewhat contemptuously, it is because
* “Insects Abroad; being a Popular Account of Foreign Insects.” By the Rev. J. G. Wood, M.A., F.L.S. London: Longman, Green, & Co. 1874.
Mr. Wood does not pretend much to philosophy; outcry for “facts” among those who regard evolunor do we think he has yet appreciated the many tion as only another name for atheism, as the surgestive hints which the new school of thought is Roman maiden demanded the brazen shields of silently working out. He is a simple narrator of the soldiers. These people know not what to do
acts, as correctly as he can make them out. These with facts when they are put into possession of are undoubtedly important if well arranged and them, and are borne down to the earth, and helpthoroughly understood. There is a pretended lessly crushed beneath the overwhelming mass of
“facts” which every day are crowding before our notice. Mr. Wood is not one of these ignorant pseudocritics: his books always strike one as thoroughly genuine. Their tone is quietly earnest, and the literary style of them all is most admirable. In his “Insects at Home,” Mr. Wood compiled a large and most useful volume of entomological reference, well written, and equally well and copiously illustrated. But the shelves of most natural history students contain works on British entomology, whereas exotic entomology has been, in England, but feebly
represented. Our museums often contain magnifi
cent specimens of tropical lepidoptera; but how rarely do we find even a few of them properly named. Then as regards many other kinds of insects, the foreign beetles, fireflies, mantids, ants, &c., the only information we have is sparsely scat
tered through books of travel, or meagrely given in
brief sketches. We hail Mr. Wood's book on foreign insects, therefore, with much pleasure, believing that a popular well-written work of this kind was much wanted by the intelligent reader; and that Mr. Wood was just the man to write it. It is a companion volume to “Insects at Home,” containing nearly 800 pages, with six hundred illustrations and full-sized plates. The letter-press is clear and pleasant to the eyes, whilst the style of wood-cutting
may be best judged of by the few blocks which have been kindly lent us for that purpose by the publishers. The larger illustration shows a group of homopterous insects, chiefly Cicadas; whilst the figures of the Eurytrachelus Titan, a beetle more than four inches in length; and the still larger “Elephant Beetle,” Megalosoma elephas—a splendid insect, black, covered with chestnut and yellow fur —will give the reader fair examples of the average merits of the wood-cuts. In fig. 36 we have a specimen of the curious order of Ambulatoria, or walkinginsects, about which so much interest hangs, from the strongly marked mimetic features they display. The illustration is that of the Eurycantha horrida—a name well deserved, for the insect is thrice the length of its portrait as here given. It is a native of New Guinea, and its eggs are said to be as large as those of the small humming-birds. A peculiar fact marks the larval stage of this insect. lf one of the limbs happen to be lost, it is immediately replaced by another. In fig. 37 we have the magnificent and well-known black and green butterfly, Papilio palinurus, whose under surface presents such a marked contrast to its upper. Fig. 38 gives us an instance of strongly-marked mimicry, not unlike that which exists in our own male “Orange-tip,” or still more, in some of the tropical “Leaf-insects.” This object is Kallima paralekta. The illustration wants some little explanation, for the artist has represented the butterfly with closed wings, at rest on a twig, one of whose leaves is mimicked by the reposing insect. This butterfly, has been already described in Wallace’s “Malayan Archipelago.” It occurs in dry woods and thickets, and so
wonderfully do its markings protect it, that even
Mr. Wallace, trained, and skilled: entomologist though he is, could not find this insect when he was pnrsuing it, and it happened to alight even near him |
The general reader will peruse this volume with much interest and pleasure; and the young naturalist will often turn to it for information that he can with difficulty procure elsewhere. Of course the author does not pretend to do other than delineate the best representatives of each order. To do more, especially in exotic entomology, would prove a task