Popular Science Monthly/Volume 27/August 1885/On Leaves II
WE have hitherto been considering, for the most part, deciduous trees. It is generally supposed that in autumn the leaves drop off because they die. My impression is that most persons would be very much surprised to hear that this is not altogether the case. In fact, however, the separation is a vital process, and, if a bough is killed, the leaves are not thrown off, but remain attached to it. Indeed, the dead leaves not only remain in situ, but they are still firmly attached. Being dead and withered, they give the impression that the least shock would detach them; on the contrary, however, they will often bear a weight of as much as two pounds without coming off.
In evergreen species the conditions are in many respects different. When we have an early fall of snow in autumn, the trees which still retain their leaves are often very much broken down. Hence, perhaps, the comparative paucity of evergreens in temperate regions, and the tendency of evergreens to have smooth and glossy leaves, such as those of the holly, box, and evergreen-oak. Hairy leaves especially retain the snow, on which more and more accumulates.
Again, evergreen leaves sometimes remain on the tree for several years; for instance, in the Scotch pine three or four years, the spruce and silver-fir six or even seven, the yew eight, A. pinsapo sixteen or seventeen, araucaria and others even longer. It is true that during the later years they gradually dry and wither; still, under these circumstances they naturally require special protection. They are, as a general rule, tough, and even leathery. In many species, again, as is the case with our holly, they are spinose. This serves as a protection from browsing animals; and in this way we can, I think, explain the curious fact that, while young hollies have spiny leaves, those of older trees, which are out of the reach of browsing animals, tend to become quite unarmed.
In confirmation of this I may also adduce the fact that while in the evergreen-oak the leaves on well-grown trees are entire and smooth edged like those of the laurel, specimens which are cropped and kept low form scrubby brushes with hard prickly leaves.
Mr. Grindon, in his "Echoes on Plant and Flower Life" (p. 30), says that "the occurrence of prickles only here and there among plants shows them to be unconnected with any general and ruling requirement of vegetation. We can only fall back upon the principle laid down at the outset, that they are illustrations of the unity of design in Nature, leading us away from the earth to Him who is 'the end of problems and the font of certainties.'" Surely, however, it is obvious that the existence of spines and prickles serves as a protection.
Another point of much importance in the economy of leaves is the presence or absence of hairs. I have already observed that most evergreens are glossy and smooth, and have suggested that this may be an advantage, as tending to prevent the adherence of snow, which might otherwise accumulate and break them down.
The hairs which occur on so many leaves are of several different types. Thus, leaves are called silky when clothed with long, even, shining hairs (silver-weed); pubescent or downy, when they are clothed with soft, short hairs (strawberry); pilose, when the hairs are long and scattered (herb-robert); villous, when the hairs are rather long, soft, white, and close (forget-me-not); hirsute, when the hairs are long and numerous (rose-campion); hispid, when they are erect and stiff (borage); setose, when they are long, spreading, and bristly (poppy); tomentose, when they are rather short, soft, and matted; woolly, when long, appressed, curly, but not matted (corn-centaury); velvety, when the pubescence is short and soft to the touch (foxglove); cobwebby, when the hairs are long, very fine, and interlaced like a cobweb (thistle, cobwebby houseleek). The arrangement of the hairs is also interesting. In some plants there is a double row of hairs along the stem. In the chickweed only one. This, perhaps, serves to collect rain and dew, and it is significant that the row of hairs is always opposite to the flower-stalk, which also has a single row. Now, the flower-stalk is for a considerable part of its life turned downward, with the row of hair outward. This, perhaps, may account for the absence of hairs on that side of the stem.
Many leaves are clothed with woolly hairs while in the bud, which afterward disappear. Thus, in the rhododendron, horse-chestnut, and other species, the young leaves are protected by a thick felt, which, when they expand, becomes detached and drops off. Many leaves are smooth on the upper side, while underneath they are clothed with a cottony, often whitish, felt. This probably serves as a protection for the stomata. In some cases the hairs probably tend to preserve the leaves from being eaten. In others, as Kerner has suggested, they serve to keep off insects-apparently with the special object of preventing Fig. 20 the flowers from being robbed of their honey by insects which are not adapted to fertilize them. Fritz Müller, to whom we are indebted for so many ingenious observations, gives an interesting case. The caterpillar of Eunomia eagrus, when about to turn into the chrysalis (Fig. 20), breaks off its hairs and fastens them to the twig which it has selected, so as to form on each side of itself about half a dozen stiff fences, to protect it during its helpless period of quiescence.
Vaucher long ago observed, though he gave no reason for the fact, that among the Malvaceæ (mallows) the species which produce honey are hairy, and those which do not are glabrous.
If we make a list of our English plants, marking out which species have honey and which have hairs, we shall find that we may lay it down as a general rule that honey and hairs go together. The exceptions, indeed, are very numerous, but when we come to examine them we shall find that they can generally be accounted for. I have made a rough list of the species in the English flora which have honey and yet are glabrous. It does not profess to be exactly correct, because there are some species with reference to which I was unable to ascertain by personal examination, or by reference to books, whether they produced honey or not. My list, however, comprised 110 species.
Now, in the first place, of these 110 species, in sixty the entrance to the honey is so narrow that even an ant could not force its way in; twenty are aquatic, and hence more or less protected from the visits of ants and other creeping insects; thus we shall frequently find that, if, in a generally hairy genus, one or more species are aquatic, they are also glabrous—as, for instance, Viola palustris, Veronica anagallis, V. beccabunga, and Ranunculus aquatilis. Polygonum amphibium is peculiarly interesting, because, as Kerner has pointed out, aquatic specimens are glabrous; while in those living on land the base of the leaf produces hairs. Half a dozen are early spring plants which flower before the ants are roused from their winter sleep; about the same number are minute ground-plants to which hairs could be no protection; three or four are night flowers; there still remain a few to be accounted for, which would have to be considered individually, but probably the evidence is sufficiently complete to justify the general inference.
Lastly, I must not omit to mention the hairs which have a glandular character.
The next point to which I would call attention is the remarkable manner in which certain forms repeat themselves. In some cases, there seems much reason to suppose that one plant derives a substantial advantage from resembling another. For instance, Chrysanthemum inodorum, the scentless mayweed, very closely resembles the camomile in leaves, flowers, and general habit. The latter species, however, has a strong, bitter taste, which probably serves as a protection to it, and of which also, perhaps, the scentless mayweed may share the advantage. These two species, however, are nearly allied to one another, and I prefer, therefore, to take as an example of mimicry the stinging-nettle (Urtica) and the common dead-nettle (Lamium album). These two species belong to totally different families; the flowers are altogether unlike, but the general habit and the form of the leaves are extremely similar.
How close the similarity is may be seen by the illustration (Fig. 21), Fig. 21. taken from an excellent photograph made for me by Mr. Harman, of Bromley. The plants on the right are true stinging-nettles; those on the left are the white dead-nettle, one of which is in flower. So close was the resemblance that, after getting the photograph, I went back to the spot on which they were growing to assure myself that there was no mistake. It can not be doubted that the true nettle is protected by its power of stinging; and, that being so, it is scarcely less clear that the dead-nettle must be protected by its likeness to the other. Moreover, though I was fortunate in lighting on so good an illustration as that shown in the figure just when I had the opportunity of photographing it, still every one must have observed that the two species are very commonly found growing together. Assuming that the ancestor of the dead-nettle had leaves possessing a faint resemblance to those of the true nettle, those in which the likeness was greatest would have the best chance of survival, and consequently of ripening seeds. There would be a tendency, therefore, according to the well-known principles of Mr. Darwin, to a closer and closer resemblance. I am disposed to suggest whether these resemblances may not serve as a protection, not only from browsing quadrupeds, but also from leaf-eating insects. On this part of the subject we have as yet, however, I think, no sufficient observations on record.
Ajuga chamæpitys, the yellow bugle, has leaves crowded and divided into three linear lobes, the lateral ones sometimes again divided. They differ, therefore, greatly from those of its allies, and this puzzled me much until one day I found it growing abundantly on the Riviera among Euphorbia cyparissias, and I was much struck by the curious likeness. The Euphorbia has the usual acrid juice of the genus, and it struck me that the yellow ajuga was perhaps protected by its resemblance.
Leaves which float on the surface of still water tend to be orbicular. The water-lilies are a well-known illustration. I may also mention Limnanthenum nymphœoides, which, indeed, is often taken for a water-lily, though it really belongs to the family of gentians, and Alisma natans, a species allied to the plantains. In running water, on the contrary, leaves tend to become more or less elongated.
Subaqueous leaves of fresh-water plants have a great tendency either to become long and grass-like or Jo be divided into more or less hair-like filaments. I might mention, for instance, Myriophyllum; Hippuris, or mare's-tail, a genus which among English plants comes Fig. 22. next to Circæa, the enchanter's nightshade; Ranunculus aquatilis a close ally of the buttercup; and many others.
Some, again, which, when mature, have rounded, floating leaves, have long, narrow ones when young. Thus in Victoria regia the first leaves are filiform, then come one or more which are sagittate, and then follow the great orbicular leaves.
Another interesting case is that in which the same species has two forms of leaf (Fig. 22)—namely, more or less rounded ones on the surface, and a second series which are subaqueous and composed of more or less linear or finely divided segments.
Mr. Grant Allen has suggested that this tendency to subdivision in subaqueous leaves is due to the absence or paucity of carbonic acid. I have ventured to suggest a different explanation. Of course it is important to expose as large a surface as may be to the action of the water. We know that the gills of fish consist of a number of thin plates, which while in water float apart, but have not sufficient consistence to support even their own weight, much less any external force, and consequently collapse in air. The same thing happens with thin, finely cut leaves. In still water they afford the greatest possible extent of surface with the least expenditure of effort in the formation of skeleton. This is, I believe, the explanation of the prevalence of this form in subaqueous leaves.
Again, in still air the conditions, except so far as they are modified by the weight, would approximate to those of water; but the more the plant is exposed to wind the more would it require strengthening. Hence, perhaps, the fact that herbs so much oftener have finely cut leaves than is the case with trees. In the Umbellifers, for instance, almost all the species have the leaves much divided—more, I need hardly say, than is the case with trees. Shrubs and trees are characterized by more or less entire leaves, such as those of the laurel, beech, hornbeam, lime, or by similarly shaped leaflets, as in the ash, horse-chestnut, walnut.
There are, however, many groups of plants which, while habitually herbaceous, contain some shrubby species, or vice versa. Let us take some groups of this description in which the herbaceous species have their leaves much cut up, and see what is the character of the foliage in the shrubby species.
The vast majority of Umbellifers, as I have just observed, are herbaceous, and with leaves much divided, the common carrot being a typical example. One European species, however, Bupleurum fructicosum, is a shrub attaining a height of more than six feet, and has the leaves (Fig. 23) coriaceous, and oblong-lanceolate.
|Fig. 23.||Fig. 24.|
The common groundsel (Fig. 24), again, is a low herb with much cut leaves. Some species of Senecio, however, are shrubby, and their leaves assume a totally different character, Senecio laurifolius and S. populifolius having, as their specific names denote, leaves respectively resembling the laurel and poplar. In the genus Oxalis, again, to which the shamrock belongs, there is a shrubby species, O. laureola, with leaves like those of a laurel.
I would venture, then, to suggest these considerations as throwing light on the reason why herbaceous plants so often have their leaves much cut up.
Next let me say a few words on the reasons why some plants have broad and some narrow leaves. Both are often found within the limits of a single genus. I have ventured to indicate the distance between the buds as a possible reason in certain cases. It would not, however, apply to herbaceous genera such as Plantago or Drosera. Now, Drosera rotundifolia (Fig. 25) has the leaves nearly orbicular,
|Fig. 25.||Fig. 26.|
while in D. anglica (Fig. 26) they are long and narrow. Plantago media (Fig. 27) has ovate leaves, while in P. lanceolata (Fig. 28) they are lanceolate, and in P. maritima nearly linear. More or less similar cases occur in Ranunculus.
These differences depend, I believe, on the attitude of the leaf, for it will be found that the broad-leaved ones are horizontal, forming a rosette more or less like that of a daisy, while the species with narrower leaves carry them more or less erect. In the daisy the rosette lies on the ground, but in other cases, as in Daphne (Fig. 29), it is at the end of a branch.
Any one who has looked with an observant eye at the vegetation of hot, dry countries must have noticed how much the general character of the vegetation differs from that which prevails in a climate like ours. There is a marked increase of prickly, leathery, and aromatic species. The first two characteristics evidently tend to protect the leaves. As regards the third, Mr. Taylor, in his charming book on "Flowers," has pointed to the power which, as Tyndall has shown, the spray of perfume possesses to bar out the passage of heat-rays, and has suggested that the emission of essential oils from the leaves of many plants which live in hot climates may serve to protect themselves against the intensely dry heat of the desert sun.
|Fig. 27.||Fig. 28.|
I am rather disposed to think that the aromatic character of the leaves protects them by rendering it less easy for animals to eat them. In still drier regions, such as the Cape of Good Hope, an unusually large proportion of species are bulbous. These, moreover, do not belong to any single group, but are scattered among a large number of very different families: the bulbous Fig. 9. condition can not, therefore, be explained by inheritance, but must have reference to the surrounding circumstances. Moreover, in a large number of species the leaves tend to become succulent and fleshy. Now, in organisms of any given form the surface increases as the square, the mass as the cube, of the dimensions. Hence, a spherical form, which is so common in small animals and plants, and which in them offers a sufficient area of surface in proportion to the mass, becomes quite unsuitable in larger creatures, and we find that both animals and plants have orifices leading from the outside to the interior, and thus giving an additional amount of surface. But in plants which inhabit very dry countries it is necessary that they should be able to absorb moisture when opportunity offers, and store it up for future use. Hence, under such circumstances fleshy stems and leaves are an advantage, because the surface exposed to evaporation is smaller in proportion than it would be in leaves of the ordinary form. This is, I believe, the reason why succulent leaves and stems are an advantage in very dry climates, such as the Canaries, Cape of Good Hope, etc.
The genus Lathyrus, the wild pea, contains two abnormal and interesting species, in which the foliaceous organs give the plant an appearance very unlike its congeners. Fig. 30 represents L. niger, with leaves of the ordinary type. In the yellow pea (L. aphaca, Fig. 31), the general aspect is very different, but it will be seen on a closer
|Fig. 30.||Fig. 31.|
inspection that the leaves are really absent, or, to speak more correctly, are reduced to tendrils, while the stipules, on the contrary, are, in compensation, considerably enlarged. They must not, therefore, be compared with the leaves, but with the stipules of other species, and from this point of view they are of a more normal character, the principal difference, indeed, being in size.
The grass pea (L. nissolia, Fig. 32) is also a small species. It lives in meadows and the grassy borders of fields, and has lost altogether, not only the leaves, but also the tendrils. Instead, however, of enlarged stipules, the functions of the leaves are assumed by the leafstalks, which are elongated, flattened, linear, ending in a fine point, and, in fact, so like the leaves of the grasses among which the plant lives that it is almost impossible to distinguish it except when in flower. For a weak plant growing among close grass, a long linear leaf is, perhaps, physically an advantage; but one may venture to suggest that the leaves would be more likely to be picked out and eaten if they were more easily distinguishable, and that from this point of view also the similarity of the plant to the grass among which it grows may also be an advantage.
In looking at foliage I have often been much puzzled as to why the leaves of some species are tongue-shaped, while others are lobed. Take, for instance, the black bryony (Tamus communis) and the common bryony (Bryonia dioica). Again, why are the veins in some leaves pinnate, like those of the beech and elm, Fig. 32. and others palmate, as in the maple and sycamore?
My first idea was that this might have reference to the arrangement of the woody fibers in the leaf-stalk. If we make a section of the stalk of a leaf, we shall find that in some cases the woody fibers are collected in the middle, while in others there are several distinct bundles, separated by cellular parenchyma. My first idea was that each of the primary ribs of a leaf might represent a separate woody fiber in the leaf-stalk, so that leaves with a single bundle of woody fibers would be pinnate; those with several distinct bundles, palmate.
The first species which I examined favored this view. The melon, geranium, mallow, cyclamen, and other species with palmate leaves, had, sure enough, several woody fibers; while, on the contrary, the laurel, rhododendron, privet, beech, box, castanea, arbutus, phillyrea, and other leaves with pinnate veins, had one central bundle. But I soon came across numerous exceptions, and had to give up the idea.
I then considered whether the difference could be accounted for by the mode of growth of the leaf, and I am still disposed to think that it has some bearing on the subject, though this requires further study.
The next suggestion which occurred to me was that it might be connected with the "prefoliation" or arrangement of the leaves in the bud. The first palmate leaves which I examined were what is called "plicate," or folded up more or less like a fan; while the leaves with pinnate veins were generally "conduplicate," or had the one half applied to the other. But, though this was true in many cases, it was not a general rule, and I was obliged to give up this idea also.
It then occurred to me to take climbing plants, and see whether I could find any relation between palmate and tongue-shaped leaves on the one hand and the mode of growth on the other—whether, for instance, the one turned generally up, the other down; whether the one were generally twining and the other clasping, or vice versa. All these suggestions one by one broke down.
Among monocotyledons, however, the tongue-shaped preponderates greatly over the palmate form of leaf. With very few exceptions, the forms of the leaves of climbing monocotyledons are in fact just such as would be obtained by widening more or less the linear, grass-like leaf which is so prevalent in the class.
This, then, raises the question whether the heart-shaped leaf is the older form from which the palmate type has been gradually evolved. Let us see whether we can find any evidence Fig. 33. bearing on this question in what may be called the embryology of plants. The furze, with its spiny prickles, belongs to a group of plants which, as a general rule, have trifoliate or pinnate leaves. Now, if we examine a seedling furze (Fig. 33), we shall find that the cotyledons are succeeded by several trifoliate leaves, with ovate leaflets. These gradually become narrower, more pointed, and stiffer, thus passing into spines. Hence, we can hardly doubt that the present furze is descended from ancestors with trifoliate leaves. I have already referred to other cases in which the young plants throw light on the previous condition of the species (ante, p. 12).
Now we shall have no difficulty in finding cases where, while in mature plants the leaves are more or less lobed and palmate, the first leaves succeeding the cotyledons are heart-shaped. This would seem to point to the fact that when in any genus we find heart-shaped and lobed leaves, the former may represent the earlier or ancestral condition.
The advantage of the palmate form may perhaps consist in its bringing the center of gravity nearer to the point of support. Broad leaves, however, are of two types: cordate, with veins following the curvature of the edge; and palmate or lobed leaves, with veins running straight to the edge. The veins contain vascular bundles which conduct the nourishment sucked up by the roots, and it is clearly better that they should hold a straight course, rather than wind round in a curve. As the nourishing fluids pass more rapidly along these vascular bundles, the leaf naturally grows there more rapidly, and thus assumes the lobed form, with a vein running to the point of each lobe.
On the whole, we see, I think, that many at any rate of the forms presented by leaves have reference to the conditions and requirements of the plant. If there was some definite form told off for each species, then, surely, a similar rule ought to hold good for each genus. The species of a genus might well differ more from one another than the varieties of any particular species; the generic type might be, so to say, less closely limited; but still there ought to be some type characteristic of the genus. Let us see whether this is so. No doubt there are many genera in which the leaves are more or less uniform, but in them the general habit is also, as a rule, more or less similar. Is this the ease in genera where the various species differ greatly in habit? I have already incidentally given cases which show that this is not so, but let us take some group—for instance, the genus Senecio, to which the common groundsel (Fig. 24) belongs, as a type well known to all of us and look at it a little more closely.
The leaves of the common groundsel I need not describe, because they are familiar to us all. This type occurs in various other species of more or less similar habit.
On the other hand, the fen Senecio(S. paludosus) and the marsh Senecio (S. palustris), Fig. 34. which live in marshy and wet places, have long, narrow, sword-shaped leaves, like those of so many other plants which are found in such localities. The field Senecio (S. campestris, Fig. 34), which lives in meadows and pastures, has a small terminal head of flowers springing from a rosette of leaves much like those of a common daisy (Bellis perennis); a Madagascar species, as yet I believe unnamed, is even more like a daisy. Senecio junceus looks much like a rush; S. hypochcerideus, of South Africa, strikingly resembles a hypochœris, as its name denotes. A considerable number of species attain to a larger size and become woody so as to form regular bushes. S. buxifolius has very much the general look of a box, S. vagans of a privet, S. laurifolius of a laurel, ericœfolius of a heath, pinifolius of a fir, or rather a yew.
Again, some species are climbers: S. scandens and S. macroglossus have leaves like a bryony; S. araneosus and S. tamoides like a smilax or (yam) tamus; S. tropœolifolius like a tropæolum.
Among the species inhabiting hot, dry regions are some with swollen, fleshy leaves, such as S. haworthii, from the Cape of Good Hope, and S. pteroneura, from Magador. Senecio rosmarinifolius, of the Cape, is curiously like a rosemary or lavender.
Lastly, some species may almost be called small trees, such as S. populifolius, with leaves like a poplar; and S. amygdaloides, like an almond.
I might mention, if space permitted, many other species which, as their names denote, closely resemble forms belonging to other groups—such, for instance, as Senecio lobelioides, erysimoides, bupleurioides, verbascifolius, juniperinus, ilicifolius, acanthifolius, linifolius, platanifolius, graminifolius, verbenefolius, rosmarinifolius, coronopifolius, chenopodifolius, lavanderiæfolius, salicifolius, mesembryanthemoides, digitalifolius, abietinus, arbutifolius, malvæfolius, erodiifolius, halimifolius, hakeæfolius, resedæfolius, hederæfolius, acerifolius, plantigineus, castaniæfolius, spiræifolius, bryoniæfolius, primulifolius, and many more. These names, however, indicate similarities to over thirty other perfectly distinct families.
It seems clear, then, that these differences have reference not to any inherent tendency, but to the structure and organization, the habits and requirements, of the plant. Of course, it may be that the present form has reference not to existing, but to ancient, conditions, which renders the problem all the more difficult. Nor do I at all intend to maintain that every form of leaf is, or ever has been, necessarily that best adapted to the circumstances, but only that they are constantly tending to become so, just as water always tends to find its own level.
But, however this may be, if my main argument is correct, it opens out a very wide and interesting field of study, for every one of the almost infinite forms of leaves must have some cause and explanation.—Contemporary Review.
- Bunbury, "Botanical Fragments," p. 320.
- Mr. Grant Allen, who had been also struck by the fact that herbaceous plants so often have their leaves much cut up, has suggested a different explanation, and thinks it is due to "the fierce competition that goes on for the carbon of the air between the small matted undergrowth of every thicket and hedge-row."
- Page 311.