ALTERNATION OF GENERATIONS. 22^ 



their growth in such a manner that all the vital phenomena of the mother-plant 

 are one by one reproduced in it. Since the same individual can form a number 

 of reproductive organs, either simultaneously or successively, so there is also, in 

 the reproductive process, at least the possibility of a multiplication of individuals, 

 inasmuch as, under favourable vital conditions, a number of descendants of the 

 same mother-plant actually come into existence. But since all those portions of 

 the surface of the globe which it is possible for plants to inhabit are already covered 

 with vegetation, it is in general only possible for such a proportion of these 

 descendants to arrive at full maturity, that the number of individuals in existence 

 remains the same from year to year. We shall see in the third book what an 

 important bearing this fact has on the struggle for existence and on the conse- 

 quent production of new vegetable forms. At present we have to consider only 

 the most important morphological phenomena connected with the organs of 

 reproduction. 



The parts which become separated for the purpose of reproduction are very 

 various in their nature. Among Cryptogams they consist most commonly of single 

 cells, — spores, gonidia (or conidia\ oospheres, anther ozoids : less often they are bodies 

 consisting of a small number of cells united into a tissue, like the gemmce or bulbils 

 of the Marchantieae. In the more highly organised plants it frequently occurs that 

 shoots, t. e. portions of the axis bearing leaves, become detached of their own accord 

 in the bud- condition, then put out roots, and continue an independent growth ; buds 

 of this kind occur, for example, in some Mosses, in many Ferns, in Lilium 

 hulUferum, several species oi Allium, &c. Very often almost any part of the plant, 

 such as detached pieces of leaves, stems, roots, &c., may become organs of reproduc- 

 tion, that is, they are able, under favourable conditions, to put out adventitious buds, 

 and thus develope into new plants. In Phanerogams, finally, the normal reproductive 

 bodies are the seeds, in which, even before separation from the mother-plant, a 

 new individual has already advanced to a lower or higher stage of development, 

 so that when the seed germinates nothing more is necessary than an increase in 

 size of the parts that are already formed, — root, stem, and leaves. 



In some cases the organs of reproduction appear, as it were, accidentally; 

 but we will not here consider these cases, but rather turn our attention to those 

 in which the formation of these organs is a necessary part of the life-history, and 

 is essential to the complete development of the plant. These normal but still 

 very various reproductive organs may be divided first of all into two groups, the 

 sexual and the asexual. 



Reproduction is said to be asexual when the part of the plant which becomes 

 detached is able, without the assistance of any other organ, to produce a new 

 individual. Of this nature are the spores of the Hymenomycetous Fungi and of 

 Ferns, the gemmae of Hepaticse, and most zoogonidia of Algae. 



Reproduction is, on the contrary, sexual when two organs, developed expressly 

 for this purpose, co-operate to produce a body out of which, either directly or after 

 some further processes, one or more new individuals arise. Notwithstanding the 

 great variety in the form of the organs of sexual reproduction in the vegetable 

 kingdom, and the complicated nature of the processes which often, especially in 

 the higher plants, precede the act of sexual union, the essential feature of this 



