225 



R ALTERNATION OF GENERATIONS. 



In some plants we find only one kind of reproduction, either only the 

 cual, as in some of the simplest Algae and Fungi, or only the sexual, as in 

 the Conjugatae. 



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 Eurotium^ 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, multiplication 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 Equisetaceae 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^ 



Q 



