CHAPTER III.

PHYSIOLOGY OF THE ORGANS OF REPRODUCTION.

HAVING now alluded to the special functions of the elementary structures, and of the organs of nutrition, and also to the general physiology or life of the whole plant, we proceed in the next place to treat of the functions of the organs of reproduction.

1. Functions of Bracts and Floral Envelopes.One of the principal offices performed by these organs is to protect the young and tender parts placed within them from injury. When green, as is commonly the case with the bracts and sepals, their color is due to the presence of chlorophyl in their component cells, and they then perform the same functions as ordinary leaves. But when of other colors than green, as is usual with the petals, and occasionally with the bracts and sepals, they appear to have, in conjunction with the thalamus, a special function to perform; which consists in the production of a saccharine substance from the amylaceous matter stored up in them. This saccharine matter is designed more especially for the nourishment of the essential organs of reproduction. In fact, a similar change takes place in the process of flowering to that which occurs in germination, where the amylaceous matters are in like manner converted into those of a saccharine nature.

During this conversion of amylaceous into saccharine matters, oxygen is absorbed in great quantities from the

COLOR OF FLOWERS.

241

atmosphere, and carbon dioxide given off in a corresponding degree. Hence, the action of the parts of the flower which are of other colors than green upon the surrounding air, under the influence of solar light, differs from that of the leaves and other green organs.

The combination which under the above circumstances takes place between the carbon of the flower and the oxygen of the air is also attended by an evolution of heat, which indeed is always the case where active chemical combination is going on.

Color of Flowers.-All the colors of flowers otherwise than green depend on bodies the nature of which is very imperfectly known, though spectroscopic analysis has done something toward grouping them into series. The changes in color which many corollas undergo are supposed to depend on the oxidation of these bodies. Most of the Boraginaceæ pass from pink to blue, from their first expansion till they are fully open; the garden Convolvulus changes from pink to a fine purple in the same period. Cultivation will effect great changes in this respect, but there is a limit to its influence. Thus the Dahlia and Tulip are naturally yellow, and under cultivation may be made to assume all shades of red, orange, and white, but no tint of blue; Geraniums and the Hydrangea will take on various shades of blue, purple, red, and white, but never a yellow. These facts led De Candolle to divide flowers in this aspect into two series—a xanthic which has yellow for its base, and a cyanic which has blue-either of which can be made red or white, but will not assume the basic color of the other. There seem to be a few exceptions to this rule; e. g., Myosotis versicolor changes from yellow in the bud to blue in the open corolla, and the Hyacinth is not unfrequently a pale yellow.

Development of the Floral Envelopes.-The manner in which the floral envelopes are developed may be shortly summed up as follows:

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THE FLORAL LEAVES.

They are subject to the same laws of development as the usual foliage leaves, and make their first appearance as little cellular processes, which grow by additions to their base or points of attachment to the axis.

The calyx is commonly developed before the corolla. When a calyx is polysepalous, or a corolla polypetalous, the component sepals or petals make their first appearance in the form of little distinct papillæ or tumors, the number of which corresponds to the separate parts of the future calyx or corolla.

When a calyx is monosepalous, or a corolla monopetalous, the first appearance of these organs is in the form of a little ring, which ultimately becomes the tube of the calyx or corolla, as the case may be. When these present lobes or teeth, as they more commonly do, these arise as little projections on the top of the ring, the number of which corresponds to the future divisions of the calyx or corolla.

All irregular calyces or corollas are regular at their first formation, the cellular papillæ from which they arise being all equal in size; hence all irregularity is produced by unequal subsequent growth.

2. Functions of the Essential Organs of Reproduction.-Sexuality of Plants.-The sexuality of plants has now been proved, and, as we have repeatedly stated, the stamens of flowering plants constitute the male apparatus, and the carpels the female. Again, while the presence of distinct sexes may thus be shown in flowering plants, both of which are necessary for the formation of perfect seed, by far the greater number of flowerless plants, in like manner, as we have seen, possess certain organs the functions of which are undoubtedly sexual.

We have already, as fully as our space will admit, described the structure of the reproductive organs of both Phanerogamous and Cryptogamous plants; we now proceed to give a general summary of the more important

conclusions which have been arrived at as regards the process of reproduction in the different divisions of plants, commencing with the Cryptogamia.

(1.) Reproduction of Cryptogamous or Acotyledonous Plants.-A. Reproduction of Thallophytes.-The sexuality of all Thallophytes has not been absolutely proved, but only concluded from analogy. Sexes have been clearly shown to exist in Algæ, Characeæ, Fungi, and Lichens. The process of reproduction in the Algæ, Fungi, and Lichens has already been noticed, but the Characeæ require a little more explanation.

Reproduction of Characea or Charas. In these plants we have two kinds of reproductive organs, called, respectively, the globule and the nucule: the former is regarded as the male organ, and the latter as the female. Fecundation takes place by the passage of the spiral spermatozoids of the globule down the canal which extends from the apex of the nucule to the central cell of the same structure, which then becomes fertilized. No free spore is, however, produced, but the nucule drops off, and after a certain period germinates, though the sexual leaf-forming plant is not directly developed, but is pre

spore.

PL

sp

FIG. 245.-Pro-embryo of Chara fragilis. sp. Germinating idg pl. The pro-embryo. At d are the rhizoids, w. w'. Primary root. g. First leaves of the second generation, or Chara proper. (After Pringsheim.)

ceded by a pro-embryo, which has, however, only a limited growth, and from it are produced at one part the rhizoids

(rootlets), and further on, as a sort of lateral branch, the Chara proper. In Nitella the production of a pro-embryo has not been definitely observed, and here the new plant seems to be formed at once from the detached nucule.

B. Reproduction of Cormophytes.-Of the sexual nature of the plants in most orders of this sub-division of the Cryptogamia there can be no doubt. The sexual organs in all are also of an analogous character, and are of two kinds, one termed an antheridium, which contains spirally wound ciliated antherozoids or spermatozoids, and is regarded as the male organ; and the other, called an archegonium or pistillidium, containing an embryonal cell or germ-cell, which is the female organ. Fecundation is supposed to be effected by the contact of a spermatozoid with the germ-cell. We have already described the structure of the reproductive organs of Cormophytes (pages 175185), both before and after fertilization; it will only be necessary, therefore, in the present place, to say a few words upon the mode in which fertilization is supposed to take place in the different natural orders included in this division of the Cryptogamia, which are here, however, arranged in the inverse order to that in which they were formerly described.

(1) Hepaticacea or Liverworts.-The general nature of the reproductive organs and method of reproduction have already been described, and may be summed up as follows: The two reproductive organs of this order, which closely resemble those of the Mosses, are termed antheridia and archegonia or pistillidia, the former representing the male sex, and the latter the female. When the antheridium bursts, it discharges a number of small cells, which also burst, and each emits a very small two-ciliated spiral spermatozoid. These spermatozoids are supposed to pass down the canal of the archegonium to the germ-cell which is situated at its bottom, which thus becomes fertilized. This cell after fertilization undergoes various important