Namely, the leaves of this first node often appear in pairs, even when the subsequent leaves of the stem are placed alternately. Here then is shown a coming together and uniting of parts which Nature later separates and places at a distance one from the other. Still more remarkable is it when the cotyledons appear like many little leaves gathered round a single axis, while the stem which gradually develops out of their midst produces the subsequent leaves singly around itself. This may be very well observed in the growth of the different kinds of pine; where a wreath of needles forms as it were a calix. As we proceed we shall be reminded of this in other similar phenomena.

17

For the present we will pass over the quite shapeless single cotyledons of plants which germinate with one leaf only.

18

Let us remark, however, that even the most leaf-like cotyledons, in comparison with the subsequent leaves of the stem, are always less developed. Above all their margin is extremely simple, with as few traces of incisions in it as there are of hairs on the surface, or of any of those vessels which are to be observed in perfect leaves.

CHAPTER II

THE DEVELOPMENT OF THE STEM-LEAVES FROM NODE TO NODE

19

We are now able to observe closely the successive formation of the leaves, as the progressive operations of Nature all take place before our eyes. Some, or many, of the leaves which now appear are often already present in the seed and lie enclosed between the cotyledons; in their folded state they are called plumules or “little feathers.” Their shape, compared with that of the cotyledons and of the future leaves, varies in different plants, but they usually differ from the cotyledons in that they are flat, delicate and formed altogether like real leaves; they become entirely green, they are attached to a visible node and their relation to the following stem-leaves can no longer be denied. They are, nevertheless, inferior to them in so far as their periphery or margin is not yet perfectly formed.

20

Henceforward the further development of the leaf progresses without pause from node to node; the vein lengthens out and the veins that branch out from it extend more or less towards the edge. These different relationships of the veins to one another are the primary cause of the manifold leaf-shapes. The leaves may appear notched, deeply-incised, or formed of many leaflets joined together, in which case they resemble perfect little twigs. The date palm affords a striking example of the simplest type of leaf developing into the most manifold forms. As the leaves succeed each other the central vein grows more and more prominent, the fan-like and yet simple leaf becomes torn and divided and an extremely compound, branchlike leaf is formed.

21

The development of the leaf-stalk keeps pace with that of the leaf. The two are either intimately connected or the stalk forms a special little stem which at long last is quite easily detached from the leaf as such.

22

That this independent leaf-stalk also has a tendency to change into the form of a leaf, is disclosed by a variety of plants, the Agrumae for example. The organization of the leaf-stalk, which for the present we will pass over, will prompt us to further considerations in the future.

23

Neither can we for the moment enter upon a closer examination of bracts and stipules. We can only observe in passing that, especially when they constitute a part of the leafstalk, they share its future transformations in many remarkable ways.

24

While the leaves owe their first nourishment principally to the more or less modified watery parts which they draw from the stem, for their increased perfection and refinement they are indebted to the light and air. The cotyledons which are formed beneath the closed seed-sheath are charged, so to speak, with only a crude sap, they are scarcely or but rudely organised and quite undeveloped. In the same way the leaves are more rudely organised in plants which grow under water than in others which are exposed to the open air. Indeed, even the same species of plant develops smoother and less intricately formed leaves when growing in low damp places, while, if transplanted to a higher region, it will produce leaves which are rough, hairy and more delicately finished.

25

So also the anastomosis of the vessels which spring forth from the larger veins, seeking each other with their ends and coalescing, and thus providing the necessary basis for the leaf-skin or cuticle, if not entirely caused by subtle forms of air, is at least very much furthered by them. If the leaves of many water-plants are thread-like or assume the form of antlers we are inclined to attribute it to a lack of complete anastomosis. The growth of the water buttercup, Ranunculus aquaticus, shows us this quite obviously, with its aquatic leaves consisting of mere thread-like veins, while in the leaves developed above water the anastomosis is complete and a connected plane is formed. Indeed, occasionally in this plant, the transition may be still more definitely observed, in leaves which are half anastomosed and half thread-like.

26

Experience has taught that the leaves draw in various kinds of air which they combine with the moisture contained within them, and there is no doubt that they bring these more refined juices back again into the stem, and so greatly promote the development of the adjacent eyes. This has been ascertained by examining the kinds of air developed in the leaves of many plants, and even in the cavities of hollow stems.

27

We observe in many plants that one node spring from the other. In the stems of the cereals, grasses and reeds, which are closed from node to node, this is obvious; but it is not so obvious in plants whose centre is hollow throughout or filled with pith, that is, with loose cellular tissue. But the supposed important functions of the pith or “marrow” being now on good ground called into question, and the impulsive and productive power once claimed for it being today attributed to the inner side of the second rind, the so-called cambium, we can now more easily understand that a more highly situated node, developing as it does from a preceding one and receiving the juices from it in a finer and more highly filtered condition, benefits from the operation of the intervening leaves and will therefore develop all the more perfectly and in its turn transmit more elaborated juices to its own leaves and eyes.

28

In so far as the fluids are in this way constantly drained away and purer ones introduced, and the plant gradually develops into a more perfect condition, it attains the end ascribed to it by Nature. At length we see the leaves perfectly developed in size and form, and soon become aware of a fresh phenomenon, which tells us that the period we have observed so far is over, and that a second one is approaching, namely that of the flower.

CHAPTER III

TRANSITION TO THE FLOWER

29

The transition to the flowering condition takes place with greater or lesser rapidity. In the latter case we shall usually notice that the stem-leaves begin to contract once more from the periphery inward, and especially to lose their manifold outer incisions. On the other hand, they tend to spread out more or less where with their lower parts they are attached to the stem. At the same time we see that the spaces between the nodes of the stem become, if not perceptibly longer, at least more slender and more delicately formed in comparison with the preceding state.

30

It has been observed that copious nourishment hinders the flowering of a plant, while moderate or even scanty nourishment accelerates it. In this we see still more clearly the function of the stem-leaves which we have already been considering. As long as there are cruder juices to be drained away, the plant must continue to develop the necessary organs to carry out this task.