Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/441

Rh VEGETABLE.] REPRODUCTION 423 II. EEPKODUCTION OF PLANTS. The various modes by which plants are reproduced may be conveniently classified in two groups, namely, vege- tative multiplication and true reproduction, the distinction between them being this, that, whereas in the former the production of the new individual may be effected by organs of the most various kinds, in the latter it is always effected by means of a specialized reproductive cell. This distinction will become apparent in the following discus- sion. il Vegetative Multiplication. The simplest case of vege- ": n tative multiplication is afforded by unicellular plants. When the cell which constitutes the body of the plant has, r jj v attained its limit of size, it gives rise to two, either by iin; division or gemmation; the two cells then grow, and at the same time become separated from each other, so that eventually two new distinct individuals are produced, each of which precisely resembles the original organism. A good example of this is to be found in the gemmation of the Yeast plant. This mode of multiplication is simply the result of the ordinary processes of growth. All plant- cells thus grow and divide at some time of their life; but, whereas in a multicellular plant the products of division remain coherent, and add to the number of the cells of which the plant consists, in a unicellular plant they luer separate and constitute new individuals. In more highly '. by organized plants vegetative multiplication may be effected - K '~ by the separation of the different parts of the body from "ere. eacn ot her, each such part subsequently developing the missing members and thus constituting a new individual. This takes place spontaneously, and in a marked manner in Mosses. The main stem gradually dies away from be- hind forwards ; the lateral branches thus become isolated, and constitute new individuals. The remarkable regenerative capacity of plant-members is largely made use of for the artificial propagation of >luc " plants. A branch removed from a parent-plant will, jJL under appropriate conditions, develop roots, and so con- stitute a new plant ; this is the theory of propagation by "cuttings." A portion of a root will similarly develop one or more shoots, and thus give rise to a new plant. An isolated leaf will, in many cases, produce a shoot and a root, in a word, a new plant ; it is in this way that Begonias, for instance, are propagated. The production of new plants from the leaves occurs also in nature, as, for instance, in the so-called " viviparous " plants, of which "is Bryophyllum calycinum (Crassulacese) and many Ferns (Nephr odium _Lastr sed Filix-mas, Asplenium Athynwni Filix-fcemina, and other species of Asplenium) are examples. An interesting case of the vegetative develop- ment of new individuals from other plant-organs is afforded by Strasburger's observations on Coelebogyne ilicifolia, Funkia ; Nothoscordum fragrans, and Citrus ; he found, toryos namely, that in these plants, an adventitious formation of ned by embryos takes place by budding from the tissue of the m S- nucellus bounding the embryo-sac. But it is in the Mosses, of all plants, that the capacity for vegetative propagation is most widely diffused. Any part of a moss, whether it be the stem, the leaves, the rhizoids, or the sporogonium, is capable, under appropriate conditions, of giving rise to filamentous protonema on which new moss- plants are then developed as lateral buds. In a large number of plants we find that provision is made for vegetative multiplication by the development of '<~diaof more or less highly specialized organs. In Lichens, for instance, there are the soredia, which are minute buds of the thallus containing both algal and fungal elements ; these are set free on the surface in large numbers, and each grows into a thallus. In the Characese there are the bulbils or " starch-stars " of Chara stelligera, which are Bulbils, underground nodes, and the branches with naked base and the pro-embryonic branches found by Pringsheim on old nodes of Chara fragilis. In the Mosses small tuberous bulbils frequently occur on the rhizoids, and in many instances (Bryum annotinum, Aulacomnion androgynum, Tetraphis pellucida, &c.) stalked fusiform or lenticular multicellular bodies containing- chlorophyll, termed gemmse, Gemmae, are produced on the shoots, either in the axils of the leaves or in special receptacles at the summit of the stem. Gemmae of this kind are produced in vast numbers in Marchantia and Lunularia among the Liverworts. Similar gemmae are also produced by the prothallia of Ferns. In some Ferns (e.g., Nephrolepis tuberosa and JV. undulata) the buds borne on the leaves or in their axils become swollen and filled with nutritive materials, con- stituting bulbils which fall off and give rise to new plants. This conversion of buds into bulbils which subserve vegetative multiplication occurs also occasionally among Phanerogams, as, for instance, in Lilium bulbiferum. But many other adaptations of the same kind occur among Phanerogams, notably among annuals. Bulbous plants, Bulbs, for instance, produce each year at least one bulb or corm conns, from which a new plant is produced in the succeeding * n ^ year. In other cases, as in the case of the Potato, tubers are developed from subterranean shoots, each of which in the following year gives rise to a new individual. In other cases, again, as in Dahlia, Thladiantha dubia, &c., tuberous swellings are found on the roots, from each of which a new individual may spring. True Reproduction. It was mentioned above that the Structure true reproduction of plants is effected by specialized cells ; of spores, these cells may be generally designated spores. The structure of a spore is essentially this : it consists of a nucleated mass of protoplasm, enclosing starch or oil as reserve nutritive material, usually enclosed by a cell-wall. In those cases in which the spore is capable of germinating immediately on its development the cell-wall is a single delicate membrane consisting of cellulose ; but in those cases in which the spore may or must pass through a period of quiescence before germination the wall becomes thickened and may consist of two layers, an inner, the endospore, which is delicate and consists of cellulose, and an outer, the exospore, which is thick and rigid, frequently darkly coloured and beset externally with spines or bosses, and which consists of cutin. In some few cases among the Fungi multicellular spores are produced ; these approximate somewhat to the gemmas mentioned above as highly specialized organs for vegetative multiplication. In some cases, particularly among the Algas and also in some Fungi (Peronosporeae, Saprolegnieae, Myxornycetes, Chytridiacese), spores are produced which are for a time destitute of any cell-wall, and are further peculiar in that they are motile, and are therefore termed zoospores ; they Zoosporei move, sometimes in an amoeboid manner by the protrusion of pseudopodia, but more frequently they are provided with one, two, or many, delicate vibratile protoplasmic filaments, termed cilia, by the lashing of which the spore is propelled through the water. The zoospore eventually comes to rest, withdraws its cilia, surrounds itself with a cell-wall, and then germinates. Spores are developed in various ways, and a prefix is in many cases added to the word " spore " to indicate the nature of the process of development. Leaving details for Spor subsequent discussion, we will now confine our attention formed to the main fact that all spores are developed in one or other either . ..i 7; 77 T J.-L r sexually of two ways, either asexually or sexually. In the former or case a single reproductive organ gives rise to cells which asexnallj are capable, each by itself, of developing into a new organism ; such an organ is an asexual reproductive organ,