Systematic botanists, however, have hitherto invariably contented themselves with observing differences of structure only; and paid little or no attention to the "why" and the "wherefore" of the differences they seized upon as being more or less important for the purpose of distinguishing species. When, however, the desirability of a more thorough knowledge of the origin of parts of plants as interpreting morphological characters was felt, developmental history began to be studied; a method strongly insisted upon by Schleiden, for example; and the most elaborate result of this method of investigation is undoubtedly Payer's Traité d'Organogénie Comparée de la Fleur, published in 1857: but if it be thought sufficient to limit the study of flowers to tracing their morphological development alone, one soon begins to see that it is far from being so, and, taken by itself, it may lead one into false interpretations, so that to the study of development must be added that of anatomy. To Ph. van Tieghem we are indebted for an elaborate treatise, entitled Recherches sur la Structure du Pistil et sur l'Anatomée Comparée de la Fleur (1871), dealing with the more minute details of floral structures. This treatise, however, still leaves much to be desired.

Besides these methods, analogy and especially teratology furnish assistance of no mean value. Here we are especially indebted to Dr. M. T. Masters for his standard work on Teratology.

Now, any one of these methods taken alone would be insufficient, and in many cases would be far from thoroughly accounting for particular points under consideration.

of

Hence to arrive at a complete interpretation of the origin every sort of structure to be found in flowers, it can only

* A German edition, Pflanzen Teratologie, ed. Dammer, 1886, has numerous additions.

be done by calling in the aid of each and all these methods to the very utmost extent possible.

Lastly, to attempt any theoretical exposition of the evolutionary history of flowers, considerable caution is required; for the causes of variation are generally so obscure, the chances of seeing them in activity so small, and experimental methods of verification well-nigh impossible, that speculations on this subject cannot altogether escape the bounds of hypothesis so as to become demonstrable facts. Hence observations which I shall make later on, with reference to the origin of existing floral structures, will not profess to be anything more than theoretical, and at most only a "working hypothesis" for future investigations.

THE STRUCTURE OF A TYPICAL FLOWER.-Before considering how the innumerable forms

of flowers deviate from one another, it is advisable to assume some typical form or plan as a preliminary basis to start from, or to which all flowers, if possible, may be referred as a standard. It would be quite possible to adopt some kind of flower as it exists in nature,

2

1

but as this would be arbitrary, Fig. 1.--Diagram of a typical flower. it may be better to take an ideal

type, and the diagram (Fig. 1) will answer the purpose, in which the outermost circle is supposed to represent a cross section of the five Sepals constituting the Calyx. The second circle is that of the five Petals of the Corolla. The third stands for the Anthers of the five Stamens superposed to the sepals; the fourth being those of five Stamens superposed to the petals. These two whorls of stamens together

constitute the Androecium. Lastly, there are represented two whorls of Carpels forming the Gynoecium † or Pistil. The outermost whorl of carpels is superposed to the sepals, the innermost to the petals.

There may be additional structures in flowers, such as disks, honey-glands, etc.; but as these, when they occur on the floral-receptacle, are merely cellular protuberances and form no part of the floral whorls proper-not being foliar in their origin-they may be omitted, especially as their position is by no means constantly the same in all flowers.‡

THE PRINCIPLES OF VARIATION.-Having thus assumed an ideal type, we may at once consider the "Principles of Variation," as I propose to call them, in accordance with which the different members of flowers can be altered; so that by means of various combinations of these principles all the flowers in the Vegetable Kingdom can be brought under this one fundamental plan.

There are five principles which require special consideration. They are usually designated by the terms Number, Arrangement, Cohesion, Adhesion, and Form.

"Number" refers to the number of whorls and the number of parts in each whorl. If two or more whorls contain the same number of parts or be multiples of one another, they are said to be "symmetrical" or "isomerous." If they differ in the number of parts they are metrical" or "anisomerous." "Arrangement" refers to the relative positions of the

66

'unsym

* Why I assume two whorls for the pistil, instead of one only, as is generally done, will be understood hereafter. I have since found that Robert Brown came to the same conclusion (Col. Works, i. 293).

+ I adopt the spelling Gynæcium for the sake of uniformity; it may be regarded as a shortened form of Gynæcæcium.

I do not here allude to certain glandular structures, which may

be the homologues of arrested organs.

different whorls, as well as of those of the individual members of the whorls with regard to each other.

"Cohesion" signifies the union of parts of any, but of the same whorl. The original or ancestral condition of the parts composing every whorl is presumed, on the principles of evolution, to have been one of entire freedom; so that the members were as completely separate or free as, for example, they are in a Buttercup. Reversions to this condition of freedom may occur, and then the process is called "dialysis;" as in the case of a polypetalous Campanula occasionally cultivated as a garden plant.

66 Adhesion " signifies the union of parts of different whorls; as well as that between the ovary and the receptacular tube, constituting the so-called inferior ovary. I regard adhesion as representing a more advanced or a more highly differentiated state than that of cohesion. Reversions may occur by "solution," which brings about a freedom of parts normally united, as in the abnormal cases of Apples, double Saxifrage, members of the Umbellifera, etc., which have all their parts perfectly free, though with inferior ovaries under ordinary circumstances.

66 Form refers to the shape of the organs; such as those of sepals and petals upon which generic characters are so often founded, the length of the filaments, and other peculiarities. If all the parts of any whorl be exactly alike, it is said to be "regular; " if not, the whorl will be "irregular."

The above five principles constitute the most important in accordance with which Nature has brought about the infinite diversity which exists in the Floral world. There are minor distinctions, hereafter to be considered, such as colours, scents, etc.; but they are of less importance in investigating the causes at work which have evolved specific and generic differences amongst flowering plants.

There is another point which may be here noticed. That a flower-bud is a metamorphosed leaf-bud is now an accepted fact; but an obvious difference between them consists in the arrested state of the axis of the former, constituting the floral receptacle; and the question arises, how has this arrest been brought about ? Like all other peculiarities of structure to be described, I would attribute the arrest primarily to the altered nature of the foliar organs on becoming members of flowers. Thus, a Fir-cone and a Buttercup are arrested branches; but when the parts of a flower are reduced in number, and instead of being in a continuous spiral are grouped in "compressed cycles," * I would then (hypothetically) attribute this further reduction of the axis, as well as other features hereafter to be described, to the irritation of insects in probing for juices, and causing nectaries to be formed. † It is the commonest thing for leafbuds to be arrested, and sometimes metamorphosed as well, by insects puncturing and depositing their eggs in them. Such may be seen on the germinal shoots of Yews, Thyme, and in certain kinds of Oak-galls, etc. In all such cases the immediate effect is the total arrest of the axis, though the leaves may be but slightly altered, as in the Yew. How the various metamorphoses of leaves into petals, etc., bas followed will be discussed later on.

It must not be forgotten, however, that the tendency to shorten the axis is primarily, in some cases, due to the altered structure of the foliar organs, as in Gymnosperms; whereby they undertake the reproductive functions. At the same time, I think insects have had a good deal to do with it, in many other phanerogams, which have but few parts to their whorls.

Each of the above principles must now be considered in detail.

*See pp. 41, 42.

+ See p. 140, seqq.