Popular Science Monthly/Volume 27/July 1885/On Leaves I
By Sir JOHN LUBBOCK.
MR. RUSKIN, in one of his most exquisite passages, has told us that "flowers seem intended for the solace of ordinary humanity: children love them; tender, contented, ordinary people love them. They are the cottager's treasure; and in the crowded town mark, as with a little broken fragment of rainbow, the windows of the workers in whose heart rests the covenant of peace." I should be ungrateful indeed did I not fully feel the force of this truth; but yet it must be confessed that the beauty of our woods and fields is due at least as much to foliage as to flowers.
In the words of the same author, "The leaves of the herbage at our feet take all kinds of strange shapes, as if to invite us to examine them. Star-shaped, heart-shaped, spear-shaped, arrow-shaped, fretted, fringed, cleft, furrowed, serrated, sinuated, in whorls, in tufts, in spires, in wreaths, endlessly expressive, deceptive, fantastic, never the same from footstalk to blossom, they seem perpetually to tempt our watchfulness and take delight in outstripping our wonder."
Now, why is this marvelous variety, this inexhaustible treasury of beautiful forms? Does it result from some innate tendency of each species? Is it intentionally designed to delight the eye of man? or have the form, and size, and texture some reference to the structure and organization, the habits and requirements, of the whole plant?
I do not propose now to discuss any of the more unusual and abnormal forms of leaves: the pitchers of Nepenthes or Cephalotus, the pitfalls of Sarracenia or Darlingtonia, the spring-trap leaves of Dionæa, the scarcely less effective though less striking contrivances in our own Drosera or Pinguicula, nor the remarkable power of movement which many leaves present, whether in response to an external stimulus, as in certain mimosas, oxalises, etc., or as a spontaneous periodic movement, such as the "sleep" of many leaves, or the nearly continuous rotation of the lateral leaflets of Desmodium. I propose, rather, to ask you to consider with me the structure, and especially the forms, of the common every-day leaves of our woods and fields.
In talking the subject over with friends, I have found a widely prevalent idea that the beauty and variety of leaves are a beneficent arrangement made specially with reference to the enjoyment and delight of man. I have, again, frequently been met by the opinion that there is some special form, size, and texture of leaf inherently characteristic of each species; that the cellular tissue tends to "crystallize," as it were, into some particular form, quite irrespective of any advantage to the plant itself.
Neither of these will, I think, stand the test of careful examination.
In the first place, let us consider the size of the leaf. On what does this depend? In herbs we very often see that the leaves decrease toward the end of the shoot, while in trees the leaves, though not identical, are much more uniform, in size.
Again, if we take a twig of hornbeam, we shall find that the six terminal leaves have together an area of about fourteen square inches, and the section of the twig has a diameter of ·06 of an inch. In the beech the leaves are rather larger, six of them having an area of perhaps eighteen inches, and, corresponding with this greater leaf-surface, we find that the twig is somewhat stouter, say ·09 of an inch. Following this up we shall find that, cœteris paribus, the size of the leaf has relation to the thickness of the stem. This is clearly shown in the following table:
Impression of Stalk below the Sixth Leaf.
stem in inches.
area of six upper
leaves in inches.
In the elm the numbers are ·11 and 34, in the chestnut ·15 and 72, and in the horse-chestnut the stem has a thickness of ·32, and the six leaves have an area often of three hundred square inches. Of course, however, these numbers are only approximate. Many things have to be taken into consideration. Strength, for instance is an important element. Thus the ailantus, with a stem equal in thickness to that of the horse-chestnut, carries a smaller area of leaves, perhaps because it is less compact. Again, the weight of the leaves is doubtless a factor in the case. Thus in some sprays of ash and elder which I examined of equal diameter, the former bore the larger expanse of leaves; but not only is the stem of the elder less compact, but the elder-leaves, though not so large, were quite as heavy, if not indeed a little heavier. I was for some time puzzled by the fact that, while the terminal shoot of the spruce is somewhat thicker than that of the Scotch fir, the leaves are not much more than one third as long. May this not perhaps be due to the fact that they remain on more than twice as long, so that the total leaf area borne by the branch is greater, though the individual leaves are shorter? Again, it will be observed that the leaf area of the mountain-ash is small compared to the stem, and it may, perhaps, not be unreasonable to suggest that this may be connected with the habit of the tree to grow in bleak and exposed situations. The position of the leaves, the direction of the bough, and many other elements would have also to be taken into consideration, but still it seems clear that there is a correspondence between thickness of stem and size of leaf. This ratio, moreover, when taken in relation with the other conditions of the problem, has, as we shall see, a considerable bearing not only on the size, but on the form of the leaf also.
The mountain-ash has been a great puzzle to me; it is, of course, a true Pyrus, and is merely called ash from the resemblance of its leaves to those of the common ash. But the ordinary leaves of a pear are, as we all know, simple and ovate, or obovate. Why, then, should those of the mountain-ash be so entirely different? May not, perhaps, some light be thrown on this by the arrangement of the leaves? They are situated some distance apart, and though, as shown in the table, they are small in comparison to the diameter of the stem, still they attain a size of fifteen square inches, or even more. Now, if they were of the same form as the ordinary pear-leaf, they would be about seven inches long by two to three in breadth. The mountain-ash, as we know, lives in mountainous and exposed localities, and such a leaf would be unsuitable to withstand the force of the wind in such situations. From this point of view, the division into leaflets seems a manifest advantage.
Perhaps it will be said that in some trees the leaves are much more uniform in size than in others. This is true. The sycamore, for instance, varies greatly; in the specimen tabulated, the stem was ·13 in diameter, and the area of the six upper leaves was sixty square inches. In another, the six upper leaves had an area of rather over one hundred inches, but in this case the diameter of the stem was ·18.
Another point is the length of the internode. In such trees as the beech, elm, hornbeam, etc., the distance from bud to bud varies comparatively little, and bears a tolerably close relation to the size of the leaf. In the sycamore, maple, etc., on the contrary, the length varies greatly.
Now, if, instead of looking merely at a single leaf, we consider the whole bough of any tree, we shall, I think, see the reason of their differences of form.
Let us begin, for instance, with the common lime (Fig. 1). The leaf-stalks are arranged at an angle of about 40° with the branch, and the upper surfaces of the leaves are in the same plane with it. The
|Fig. 1.||Fig. 2.|
result is, that they are admirably adapted to secure the maximum of light and air. Let us take, for instance, the second or third leaf in Fig. 1. They are four and a half inches long and very nearly as broad. The distance between the two leaves on each side is also just four and a half inches, so that they exactly fill up the interval. In Tilia parvifolia the arrangement is similar, but leaves and internodes are both less, the leaves, say, one and a half inch, and the internodes ·6.
In the beech, the general plane of the leaves is again that of the branch (Fig. 2), but the leaves themselves are ovate in form, and smaller, being only from two to three inches in length. On the other hand, the distance between the internodes is also smaller, being, say, one and a quarter inch against something less than two inches. The diminution in length of the internode is not, indeed, exactly in proportion to that of the leaf, but, on the other hand, the leaf does not make so wide an angle with the stem. To this position is probably due the difference of form. The outline of the basal half of the leaf fits neatly to the branch, that of the upper half follows the edge of the leaf beyond, and the form of the inner edge being thus determined decides the outer one also.
In the nut (Corylus), the internodes are longer and the leaves correspondingly broader. In the elm (Ulmus, Fig. 3), the ordinary branches have leaves resembling, though rather larger than, those of the beech; but in vigorous shoots the internodes become longer and the leaves correspondingly broader and larger, so that they come nearly to resemble those of the nut.
But it may be said the Spanish chestnut (Castanea vulgaris, Fig. 4) also has alternate leaves in a plane parallel to that of the branch, and
|Fig. 3.||Fig. 4.||Fig. 5.|
with internodes of very nearly the same length as the beech. That is true; but, on the other hand, the terminal branches of the Spanish chestnut are stouter in proportion. Thus, immediately below the sixth leaf, the chestnut-stalk may be ·15 of an inch in thickness, that of the beech not much more than half as much. Consequently, the chestnut could, of course supposing the strength of the wood to be equal, bear a greater weight of leaf; but, the width of the leaf being determined by the distance between the internodes, the leaf is, so to say, compelled to draw itself out. In Fig. 5 I have endeavored to illustrate this by placing a spray of beech over one of Spanish chestnut. Moreover, not only do the leaves on a single twig thus admirably fit in with one another, but they are also adapted to the ramification of the twigs themselves. Fig. 6 shows a bough of beech seen from above, and it w 7 ill be observed that the form of the leaves is such that, while but little space is lost, there is scarcely any overlapping. Each fits in perfectly with the rest.
The leaves of the yew (Fig. 7) belong to a type very different from those which we have hitherto been considering. They are long, narrow, and arranged all round the stem, but spread right and left,
|Fig. 6.||Fig. 7.||Fig. 8.|
so that they lie in one plane, parallel to the direction of the branchlet, and their width bears just such a relation to their distance apart that when so spread out their edges almost touch. Fig. 8 represents a sprig of box. It will be observed that the increase of width in the leaves corresponds closely with the greater distance between the points of attachment.
The leaves of the Scotch pine (Pinus sylvestris) are needle-like, one and a half inch in length and one twentieth in diameter. They are arranged in pairs, each pair inclosed at the base in a sheath. One inch of stem bears about fifteen pairs of leaves. Given this number of leaves in such a space, they must evidently be long and narrow. If I am asked why they are longer than those of the yew, I would suggest that the stem, being thicker, is able to support more weight. In confirmation of this, we may take for comparison the Weymouth pine, in which the leaves are much longer and the stalk thicker.
When we pass from the species hitherto considered to the maples (Fig. 11), sycamores, and horse-chestnuts (Figs. 9 and 10), we come to a totally different type of arrangement. The leaves are placed at right angles to the axis of the branch instead of being parallel to it, have long petioles, and palmate instead of pinnate veins. In this group the mode of growth is somewhat stiff; the main shoots are perpendicular, and the lateral ones nearly at right angles to them. The buds, also, are comparatively few, and the internodes, consequently, at greater distances apart, sometimes as much as a foot, though the two or three at the end of a branch are often quite short. The general habit is shown in Figs. 9 and 10. Now, if we were to imagine six beech or
|Fig. 9.||Fig. 10.|
elm leaves on these three internodes, it is obvious that the leaf surface would be far smaller than it is at present. Again, if we compare the thickness of an average sycamore-stem below the sixth leaf with that of a beech-stem, it is obvious that there would be a considerable waste of power. Once more, if the leaves were parallel to the branch, they would, as the branches are arranged, be less well disposed with reference to light and air. A glance at Figs. 9, 10, and 11, however, will show how beautifully the leaves are adapted to their changed conditions. The blades of the leaves of the upper pair form an angle with the leaf-stalks, so as to assume a horizontal position, or nearly so; the leaf-stalks of the second pair decussate with those of the first, and are just so much longer as to bring up that pair nearly, or quite, to a level with the first; the third pair decussate with the second, and are again brought up nearly to the same level, and immediately to the outside of the first pair. In well-grown shoots there is often a fourth pair on the outside of the second. If we look at such a cluster of leaves directly from in front, we shall see that they generally appear somewhat to overlap; but it must be remembered that in temperate regions the sun is never vertical. Moreover, while alternate leaves are more convenient in such an arrangement as that of the beech, where there would be no room for a second leaf, it is more suitable in such cases as the sycamores and maples that the leaves should be opposite, because, if, other things remaining the same, the leaves of the sycamore were alternate, the sixth leaf would require an inconvenient length of petiole.
Perhaps it will be said that the plane-tree, which has leaves so like a maple that one species of the latter genus is named after it (Acer platanoides), has, nevertheless, alternate leaves. In reality, however, I think this rather supports my argument, because the leaves of the plane, instead of being at right angles to the stem, lie more nearly parallel with it. Moreover, as any one can see, the leaves are not arranged so successfully with reference to exposure as those of the species we have hitherto been considering, perhaps because, living as it does in more southern localities, the economy of sunshine is less important than in more northern regions.
The shoot of the horse-chestnut is even stouter than that of the sycamore, and has a diameter below the sixth leaf of no less than three tenths of an inch. With this increase of strength is, I think, connected the greater size of the leaves, which attain to as much as eighteen inches in diameter, and this greater size, again, has perhaps led to the dissection of the leaves into five or seven distinct segments, each of which has a form somewhat peculiar in itself, but which fits in admirably with the other leaflets. However this may be, we have in the horse-chestnut, as in the sycamores and maples, a beautiful dome of leaves, each standing free from the rest, and expanding to the fresh air and sunlight a surface of foliage in proportion to the stout, bold stem on which they are borne.
Now, if we place the leaves of one tree on the branches of another, we shall at once see how unsuitable they would be. I do not speak of putting a small leaf such as that of a beech on a large-leaved tree such as the horse-chestnut; but if we place, for instance, beech on lime, or vice versa, the contrast is sufficiently striking.
The lime-leaves would overlap one another, while, on the other hand, the beech-leaves would leave considerable interspaces. Or let us in the same way transpose those of the Spanish chestnut (Castanea) and those of Acer platanoides, a species of maple. I have taken specimens in which the six terminal leaves of a shoot of the two species occupy approximately the same area. Figs. 4 and 11 show the leaves in their natural position, those of Castanea lying along the stalk, while those of Acer are ranged round it. In both cases it will be seen that there is practically no overlapping, and very little waste of space. In Castanea the stalks are just long enough to give a certain play to the leaves. In Acer they are much longer, bringing the leaves approximately to the same level, and carrying the lower and outer ones free from the upper and younger ones.
Now, if we arrange the Spanish-chestnut leaves round a center, as in Fig. 12, it is at once obvious how much space is wasted. On the other hand, if we attach the leaves of the Acer to the stalk of Castanea at the points from which the leaves of Castanea came off, as in Fig. 13,
|Fig. 12.||Fig. 13.|
we shall see that the stalks are useless, and even mischievous, as a cause of weakness and of waste of space; while, on the other hand, if we omit the stalks, or shorten them to the same length as those of Castanea, as in Fig. 14, the leaves would greatly overlap one another.
Once more, for leaves arranged as in the beech the gentle swell at the base is admirably suited; but in a crown of leaves, such as those of the sycamore, space would be wasted, and it is better that they should expand at once as soon as their stalks have borne them free from those within. Moreover, the spreading lobes leave a triangular space (Fig. 11) with the insertion of the stalk at the apex, which seems as if expressly designed to leave room for the pointed end of the leaf within.
Hence we see how beautifully the whole form of these leaves is adapted to the mode of growth of the trees themselves and the arrangement of their buds.
Before we proceed to consider the next series of species to which I wish to direct attention, it will be necessary for me to say a few words on the microscopical structure of the leaf. Although so thin, the leaf consists of several layers of cells. Speaking roughly, and as a general rule, we may say that on each side is a thin membrane, or epidermis, underneath which on the upper side are one or more layers of elongated cells known from their form as "pallisade-cells," beneath which is a parenchymatous tissue of more or less loose texture. The leaf is strengthened by ribs of woody tissue. From this general type there are, of course, numerous variations. For instance, some water-plants have no epidermis.
If the surface of the leaf be examined with a tolerably high power, small opaque spots will be observed, resembling a sort of button-hole, with a thick rim or border composed of two more or less curved cells, the concavities being turned inward. When dry, they are nearly straight, and lie side by side; but when moistened they swell, become somewhat curved, and gape open.
It is difficult to realize the immense number of these orifices or "stomata" which a single bush or tree must possess when we remember that there are sometimes many thousand stomata to a square inch of surface. In a large proportion of herbs the two sides of the leaf are under conditions so nearly similar that the stomata are almost equally numerous on the upper and on the lower side. In trees, however, as a general rule, they are found exclusively on the under side of the leaf, which is the most protected; they are thus less exposed to the direct rays of the sun, or to be thoroughly wetted by rain, so that their action is less liable to sudden and violent changes.
There are, however, some exceptions; for instance, in the black poplar the stomata are nearly as numerous on one side of the leaf as on the other. Now, why is this? If we compare the leaves of the black and white poplar, we shall be at once struck by the fact that, though these species are so nearly allied, the leaves are very different. In the white poplar (Populus alba), the upper and under sides are very unlike both in color and texture, the under side being thickly clothed with cottony hairs. In the black poplar (P. nigra, Fig. 15), the upper and under surfaces are, which is not frequent, very similar in color and texture. The petioles or leaf-stalks, again, are unlike; those of P. nigra presenting the peculiarity of being much flattened at the end toward the leaf. The effect of the unusual structure of the petiole is that the leaf, instead of being horizontal as in the P. alba and most trees, hangs vertically, and this again explains the similarity of the two surfaces, because the result is that both surfaces are placed under nearly similar conditions as regards light and air. Again, it will be observed that, if we attempt to arrange the leaves of the black poplar on one plane, they generally overlap one another; the extent is larger than can be displayed without their interfering with one another. In foliage arranged like that, for instance, of the beech, elm, sycamore, or, in fact, of most of our trees, this would involve a certain amount of waste; but in the black poplar, as Fig. 15 shows, the leaves when hung in their natural position are quite detached from one another.
Another interesting case of a species with vertical leaves is the prickly lettuce (Lactuca scariola), while those of L. muralis and L. virosa are horizontal. With this position of the leaves is connected another peculiarity, especially well marked in the so-called "compass" plant of the American prairies (Silphium laciniatum), a yellow composite not unlike a small sunflower, which is thus named because the leaves turn their edges north and south. This has long been familiar to the hunters of the prairies, but was first mentioned by General Alvord, who called Longfellow's attention to it, and thus inspired the lines in "Evangeline":
"Look at this delicate plant, that lifts its head from the meadow,
See how its leaves are turned north, as true as the magnet;
This is the compass-flower, that the finger of God has planted
Here in the houseless wild to direct the traveler's journey
Over the sea-like, pathless, limitless waste of the desert."
The advantage of this position, and consequently the probable reason for its adoption, is that in consequence of it the two faces of the leaf are about equally illuminated by the sun; and in connection with this we find that the structure of the leaf is unusual in two respects. The stomata are about equally abundant on both surfaces, while pallisade-cells, which are generally characteristic of the upper surface, are in this species found on the lower one also.
The leaves of the Lactuca scariola have also, when growing in sunny situations, a tendency to point north and south. Under such circumstances also they have a layer of pallisade-cells on each side.
Hitherto I have dealt with plants in which one main consideration appears to be the securing as much light and air as possible. Our English trees may be said as a general rule to be glad of as much sun as they can get. But a glance at any shrubbery is sufficient to show that we can not explain all leaves in this manner, and in tropical countries some plants at any rate find the sun too much for them. I will presently return to the consideration of the general characteristics of tropical vegetation. In illustration, however, of the present point, perhaps the clearest evidence is afforded by some Australian species, especially the eucalypti and acacias. Here the adaptations which we meet with are directed, not to the courting, but to the avoidance, of light.
The typical leaves of acacias are pinnate, with a number of leaflets. On the other hand, many of the Australian acacias have leaves (or, to speak more correctly, phyllodes) more or less elongated or willow-like. But if we raise them from seed we find, for instance, in Acacia salicina, so called from its resemblance to a willow, that the first leaves are pinnate (Fig. 16), and differ in nothing from those characteristics of the genus. In the later ones, however, the leaflets are reduced in number, and the leaf-stalk is slightly compressed laterally. The fifth or sixth leaf, perhaps, will have the leaflets reduced to a single pair, and the
|Fig. 16.||Fig. 17.|
leaf-stalk still more flattened, while, when the plant is a little older, nothing remains except the flattened petiole. This in shape, as already observed, much resembles a narrow willow-leaf, but flattened laterally, so that it carries its edge upward, and consequently exposes as little surface as possible to the overpowering sun. In some species the long and narrow phyllodes carry this still further by hanging downward,
|Fig. 18.||Fig. 19.|
and in such cases they often assume a cimeter-like form. This I would venture to suggest may be in consequence of one side being turned outward, and therefore under more favorable conditions.
In one very interesting species (Acacia melanoxylon, Fig. 17), the plant throughout life produces both forms, and on the same bough may be seen phyllodes interspersed among ordinary pinnate leaves, the respective advantages being, it would appear, so equally balanced that sometimes the one, sometimes the other, secures the predominance.
In the case of the eucalyptus, every one who has been in the south of Europe must have noticed that the young trees have a totally different aspect from that which they acquire when older. The leaves of the young trees (Fig. 18) are tongue-shaped, and horizontal. In older ones, on the contrary (Fig. 19), they hang more or less vertically, with one edge toward the tree, and are cimeter-shaped, with the convex edge outward, perhaps for the same reason as that suggested in the case of acacia. There are several other cases in which the same plant bears two kinds of leaves. Thus, in some species of juniper the leaves are long and pointed, in others rounded and scale-like. Juniperus chinensis has both.
In the common ivy the leaves on the creeping or climbing stems are more or less triangular, while those of the flowering stems are ovate-lanceolate, a difference the cause of which has not, I think, yet been satisfactorily explained, but into which I will not now enter.—Contemporary Review.