As another curious instance a remarkable form of the Sun-dew, Drosera rotundifolia, may be alluded to here, as throwing additional light upon the origin of ovules. It has been described and figured by Naudin,* and also by Planchon. In this monstrosity the ovular coats were represented by "tentacles." These, as is well known, are not epidermal trichomes, but structures issuing from branches arising from the fibro-vascular cords of the leaf, and are therefore strictly homologous with the "funnels" on cabbage leaves.‡

The conclusion, therefore, which seems deducible from the foregoing observations is that an ovule is simply an appendage (not a bud) to the fibro-vascular cord of the margin of the carpel, and under monstrous conditions can grow into foliaceous excrescences to the carpellary leaf. It is not, therefore, axial in its character. Since all that is required to start from is a fibro-vascular cord, this may be furnished by any cord, even the mid-rib; and such is the case in some monstrous states of Primula, in which rudiments of ovules are found on the mid-ribs as well as on the margins of separate carpels.

As the "funnels" on the mid-ribs and lateral veins of cabbage leaves are due to an abnormal condition of hypertrophy, so ovules I consider as arising in a similar way, and take them to be due to the same influence, though of course it is normal in their case. The very presence of the large cords running up the margins of carpellary leaves, direct from the axis below,-being often, indeed, larger than the dorsal cord,-which then ramify, not only into each ovule, but often backwards within the carpellary walls till they reach and

* Ann. des Sci. Nat., 2° sér., vol. xiv., p. 14.

† Ibid., 3e sér., vol. ix., p. 86, tab. 5, 6.

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The pitchers " of Nepenthes, perhaps, originate in much the same way, from the original water-gland at the apex of the leaf.

anastomose with the dorsal cord; these, together with the greatly thickened cellular margins now constituting the placentas which supply the conducting tissue for the pollentubes, all show a form of hypertrophy in the edges of the carpellary leaves, a condition of things widely different from the usually thin and more or less impoverished margins of true leaves.

If we may recognize a fibro-vascular cord as the "fundamental unit," and as a basis for the construction of any organ, and moreover as also containing within it potentially the power of evolving any number of similar organs by repeatedly branching; then, when hypertrophy affects such a "unit," it may branch once, twice, or any number of times, when each branch passing off to the surface can lay the foundation of a repetition of the organ from which it takes its rise.* Attention has already been called to this origin of the numerous stamens of the Malvacea, and how certain forms

of double flowers originate from the multiplication of the petaline cords, each branch of which issues in a distinct petal, as in Snowdrops.

Similarly for carpels and ovules, the process of multiplication can be witnessed both normally and abnormally. On the Fig. 88.-Multifold carpels one hand, that of carpels into five groups

with ovuliferous margins

rose.

from a malformed Prim- occurs in the Hollyhock through the chorisis of the original carpellary cord; on the other, Fig. 88 represents a multifold carpel of a Primrose, due, there is no doubt, to a like chorisis of the cords belonging to one individual carpel.

Similarly for ovules, while two only are normally charac

* I must again remind the reader that I am here speaking metaphorically; as we do not know wherein this potentiality really lies, but can only describe what is actually visible.

teristic of the Plum, and Orchis has an innumerable quantity arising by repeated chorisis of the original placentary cords, so in monstrous Primroses, etc., as represented in Fig. 88, many additional ovular processes may be formed, not only on the margins, but even, as stated, on the mid-ribs.

One other point may be here noticed, à propos to the following curious discovery by M. Baillon. I have regarded a vascular cord as a "unit," as being capable of giving rise to any appendage whatever; and as long as it is in an axis as a "trace," the cords of all organs are absolutely indistinguishable. Further, there is no difference between a cord which will enter an appendage and one which will form a pedicel from a peduncle. In the latter case, several cords are usually required for the pedicel; while one, the most external of the "horseshoe " group given off at one side of the peduncle (i.e. as seen in a transverse section), enters the bract. In Erodium cicutarium, which has three flowers to the umbel with very slender pedicels, one single cord is all that the peduncle contributes to supply each of the pedicels, and one very small cord for a bract. The cord for the pedicel increases by radial chorisis, and so passes from the form of a wedge to that of a fan, when the outermost parts increase till they meet, and so a circle is established.

This shows that an "axis" and an "appendage" are fundamentally due to the same kind of “unit.'

The reader will now see that in the following case ** the funicular cord, which is normally that of a foliar, i.e. appendicular organ, supplied an axial cord; just as many leaves can give rise to buds which are often utilized for propagative purposes.

* Sur le Développement et la Germination des Grains Bulbiformes des Amaryllidées, par. M. Baillon, Bull. Soc. de Fr., t. xxi., 30 (pub. en Revue des Cours Sci. Lyon, Août 30, 1873).

"Les bulbilles des Amaryllidées ne sont pas toujours des graines véritables modifiées seulement quant à l'épaisseur et à la consistance de leurs diverses couches naturelles, notamment des plus extérieures. Témoin le Calostemma Cunninghamii. Ici, par une singulière transformation de l'ovule en bulbe, la chalaze, en s'épaississant, joue le rôle d'un véritable plateau, sur lequel se produisent une, puis plusieurs racines adventives. Les enveloppes ovulaires tienne alors lieu d'écailles bulbaires tandis qu'il s'élève dans le sac embryonnaire un véritable bourgeon parti de la chalaze comme support et s'échappant par son sommet de la cavité ovulaire pour se comporte ensuite comme une plante complète."

An analagous case of bulbs arising from a foliar organ occurred with Scilla Sibirica. Some plants dug up in October, 1887,* were found to have taken the form of the so-called droppers" not uncommon in tulips. Their peculiarity

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resides in the fact that the tubular leaf-sheath bends and grows downwards, thereby carrying the axillary bulbil to a greater depth in the soil than usual. In February, 1888, on re-examining them, Dr. Masters discovered that from one to four bulbils had been developed at different heights within the tissue of the tubular sheath, being in connection with the cords of the latter by means of a transverse nexus of tracheids.

I refer to these cases as being curious instances of axial structures proceeding from foliar-i.e., by the change of character of the fibro-vascular cords from being at first foliar and then axial. They support the theory of homology of leaf and axis, which is, of course, otherwise quite efficiently substantiated by such plants as Xylophylla, Ruscus and the Cactaceae.

* Gard. Chron. for Oct. 15, 1887, p. 475, fig. 98, "droppers;" also, for March 3, 1888, p. 276, fig. 45, ditto, with bulbils.

CHAPTER XXXI.

THE VARIETIES OF FERTILISATION.

THERE are at least seven kinds of union :- (1) self-fertilisation, or the fertilisation of a pistil by the pollen from the same flower (Autogamy); (2) crossing different flowers on the same plant; (3) crossing flowers on different plants of the same stock; (4) crossing flowers of different plants, but of different stocks; all the preceding being of exactly the same form or variety of species; (5) crossing varieties of the same species; (6) crossing different species of the same genus; (7) crossing different genera of the same order.

When a knowledge of the floral sexes was first acquired, the idea maintained was that hermaphrodite flowers were specially adapted for self-fertilisation; but it was, I believe, Dean Herbert who first observed the importance of crossing, in his work on the Amaryllidaceae (1836). He says, "I am inclined to think that I have derived advantage from impregnating the flowers from which I wished to obtain seeds with pollen from another individual of the same variety, or, at least, from another flower rather than its own, and especially from an individual grown in a different soil or aspect."

Mr. Darwin's work, On the Cross and Self Fertilisation of Plants (1876), placed on a scientific basis, by means of experimental verifications, the exact values of such crossings. His conclusions, however, require considerable modifications.