ALTERNATION OF GENERATIONS. 
225 
In some plants we find only one kind of reproduction, either only the 
asexual, as in some of the simplest Algae and Fungi, or only the sexual, as in 
the Conjugatse. 
But in most plants reproductive organs of both kinds, sexual and asexual, 
are produced either simultaneously or successively. The two kinds of reproduction 
may then occur in the same individual, as in Vaucheria or Eurotiu7n, or may be 
distributed on different individuals. In both cases the entire process of development 
may be divided into two sharply separated stages : — At the termination of one 
stage sexual organs are formed: by their union the second stage of develop- 
ment is rendered possible, and this closes with the production of asexual spores. 
Such a course of development is termed, from the analogy of certain processes 
in the animal kingdom, an Alternation of Generations^ a term which is especially 
applicable in those cases in which, in one or both of the two stages of develop- 
ment, multipUcation of the individual also takes place by gonidia or gemmae, 
so that each of the two stages is complete in itself as a sexual or asexual 
generation. 
Since we have here to do with phenomena which are foreign to ordinary life, 
and hence somewhat difficult to understand, we will illustrate the nature of alternation 
of generations by a few simple examples. 
An alternation of generations is very evident in Ferns. The plant which we call 
in common language the Fern is merely the second stage in the process of develop- 
ment of the plant, or the asexual generation [sporophore]. It consists of a stem which 
forms true leaves and roots; on the leaves are produced small capsules or sporangia, 
in which the spores are produced without any sexual process. But each of these 
spores does not, on germination, again produce a Fern, but a minute plant of 
extremely simple structure which nourishes itself independently as a leaf-like thallus 
with root-hairs. This little plant, termed a prothallium^ may, under certain circum- 
stances, reproduce itself by gemmae ; and thus from a single spore an entire genera- 
tion of prothallia will arise, which also behave as independent plants, although 
each prothallium is only the first stage of development, the sexual generation 
[oophore], of a Fern. For, finally, the prothallia produce sexual organs of repro- 
duction ; and from the oosphere of the female organ is produced an embryo 
which developes into a Fern with true roots and leaves. In this stage also 
the Fern is capable of immediate multiplication by the production of bulbils 
from which Fern-plants are directly developed, but the normal development closes 
with the production of spores. Precisely the same processes as in Ferns take 
place also in the Equisetacese and Ophioglossaceae. In the Selaginelleae the pro- 
thallium is formed inside the spore, and this class therefore establishes a transition 
to Phanerogams, where the prothallium .is altogether rudimentary, and is found in a 
spore-like structure, the embryo-sac, within the ovule; so that the alternation of 
generations, so evident in Ferns, can be recognised here only by the most careful 
comparison with the most highly developed Cryptogams. This will be explained 
more in detail in Book II. 
In the Muscineae the alternation of generations is no less clear than in Ferns, 
although it assumes an entirely different form. A Moss, in the state in which we 
ordinarily see it, consists of a stem provided with numerous leaves and root-hairs ; 
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