The Metamorphosis of Plants (5 page)

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.
If superfluous nourishment is forced on the plant, this task must be continued, and flowering becomes almost impossible.
But if excessive nourishment is withheld, Nature's operation is rather hastened and facilitated, the organs of the nodes become more refined, the unadulterated juices act more purely and more strongly.
In a word, the metamorphosis of parts is made possible and takes place without delay.

We often see this change taking place quickly; when this is so the stem shoots upward all at once from the node of the last developed leaf and becomes tapering and more delicate, ending in a little collection of leaves around an axis.

There seems to us to be quite clear proof that the leaves of the calyx are the same organs as those which we have so far seen developing into stem-leaves, only now they are collected—often in a very changed form—round a common centre.

We have already noticed a similar operation of Nature in the cotyledons, where several leaves—nay more, obviously several nodes—are gathered close to one another round a single point.
The pine species in their development from the seed show a rayed circle of unmistakable needles which, in comparison with other cotyledons, are highly developed.
When this plant is still quite young we can already see an indication, as it were, of that force of Nature which at a more advanced age will produce the blossom and fruit.

Furthermore, in many flowers we see unaltered stem-leaves collected together so as to form a kind of calyx just below the flower.
As their form is still quite unchanged we can recognise that they are leaves by their appearance, and indeed in the botanical terminology they are called “flower leaves”—
folia floralia.

With great attention we must watch the procedure in the case already mentioned when the transition to the flowering period takes place slowly; the stem-leaves gradually draw together, become modified and pass almost unawares into calyx-leaves.
This may readily be seen in the compositae, especially in sunflowers and marigolds.

This force of Nature which collects a number of leaves around a single axis can bring about a still more intimate union, making the clustered and modified leaves more than ever difficult to recognise.
The calyx-leaves or sepals are then joined together—either entirely so, or only partly grown together at the edges.
These leaves, crowded and closely pressed to one another, touch most intimately while in their tender state.
They become anastomosed under the influence of the very pure juices now present in the plant, and form the bell-shaped or so-called one leaved calyces revealing their composite
origin by the way in which they are more or less incised or divided.
We shall learn this if we compare a number of deeply incised single calyces with many-leaved ones and especially if we observe the calyces or involucres of some compositæ.
Thus, for example, we shall see that the calyx of a marigold, which is defined in systematic descriptions as “simple and much-divided,” consists of several leaves grown into and over one another, into which, as we have already said, the contracted stem-leaves imperceptibly pass over.

In many plants the number and form in which the calyx-leaves or sepals, whether single or grown together, are arranged around the axis of the stalk, is constant, and this is also the case with the other subsequent parts.
On this constancy rests to a great extent the progress, certainty and reputation of botanical science, which in recent years has been making continual advances.
In other plants, the number and formation of these parts is not so constant, yet even this inconstancy has not escaped the keen powers of observation of the masters of this science; on the contrary, they have tried, by means of exact definitions, to restrict even this variation of Nature, as it were into some pattern of conformity.

Thus has Nature formed the calyx, by uniting—around a common centre, and as a rule in definite number and order—many leaves and consequently many nodes which she would otherwise have produced one after the other and at some distance apart.
If the flowering period had been retarded by the in-streaming of superfluous nourishment, the nodes and leaves would have appeared separated from one another in their original form.
Nature, therefore, in forming the calyx creates no new organ, but simply combines and modifies the organs we already know, advancing in this way a step nearer her goal.

We have seen how the calyx is produced through the influence of refined juices gradually generated in the plant, and now the calyx itself is destined to become the organ of a future and further degree of refinement.
We can believe this even if we explain its operation from a purely mechanical point of view.
How tender and capable of the finest filtration must be the vessels which are so highly contracted and drawn together!

The transition from the calyx to the corolla can be seen quite clearly, for although the calyx is usually green like the stem-leaves, it often shows a change in one part or another at the tips, the edges, the back or even over the inner surface, leaving the outer surface green.
Also whenever this colouring occurs we see it combined with an increased refinement of texture.
Thus there arise the calyces which we should be equally justified in regarding as corollas.

We have observed from the seed-leaves (cotyledons) upward a process of great expansion and development of the leaves to the periphery, while in the transition to the calyx we see once more a contraction from the circumferance towards the centre.
We now notice that the corolla is produced by yet another expansion.
The petals are usually larger than the calyx-leaves or sepals.
Even as the organs were contracted into the calyx, so do they now expand again into petals under the influence of the still more finely filtered juices which have passed through the calyx to appear in a highly refined state as new and quite different organs.
Their delicate organisation, their colour and their scent would make it quite impossible to recognise their origin if we were not able to hearken to Nature as she speaks to us through her many vagrancies and abnormalities.

Thus, for instance, inside the calyx of a carnation a second calyx is often found which on the one hand, inasmuch as part of it is quite green, reveals its tendency to become a one-leaved, incised calyx, while on the other hand it is tom and jagged and beginning at the tips and edges to expand and to become tinted like the real petals.
Through this we clearly recognise the relationship between the corolla and the calyx.

The relation of the corolla to the stem-leaves reveals itself in different ways.
On many plants the stem-leaves are produced more or less coloured long before they approach the flowering state; in other cases they become completely coloured when they get near to the flower.

Sometimes, too, Nature proceeds immediately to the corolla, omitting the calyx altogether and we are given the opportunity of observing the transformation of stem-leaves into petals.
On tulip stalks, for example, an almost perfectly formed and coloured petal may sometimes be seen.
Indeed it is even more remarkable when such a leaf, half green and half coloured, belongs with its green part to the stem and remains attached thereto, while its coloured part is carried up into the corolla so that the leaf is torn in two.

It is a not unlikely opinion which would ascribe the colour and scent of the petals to the presence of the male seed within them.
It may be there in an insufficiently separated state, combined with and diluted by other juices.
The mani-

fold and beautiful appearances of colour incline us to the thought that the substance contained in the petals, although
it is in an extremely purified condition, has not yet attained the very highest degree of purity, which would be white, absolutely without shade or colour.