A L TER NA TION OF GENERA TIONS. 
223 
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 gemmcB or bulbils 
of the Marchantieae. In the more highly organised plants it frequently occurs that 
shoots, i. 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 Liliian 
bulbi/erum, several species of 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 gemmse of Hepaticae, 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 
