290 BOTANY 



considered as an example of the alternation of generations. This 

 expression has been restricted to cases where there is a regular 



ALTERNATION BETWEEN A VEGETATIVE AND SEXUAL GENERATION, EACH 

 OF WHICH HAS AN ENTIRELY DIFFERENT ORGANISATION. 



A Fern-plant produces only asexual spores. By their germination, however, a 

 Fern-plant is not produced, but in its place a diminutive plantlet, which remains 

 without stem and leaves, without vascular bundles, and without any internal 

 differentiation. This is the prothallium, which in turn produces sexual organs 

 with spermatozoids and egg-cells, from which a large Fern-plant is developed after 

 fertilisation. In a similar manner, sexual and asexual generations alternate in 

 the Mosses and in the Hydropteridme, Mquisetinae, Lycopodinae. In the three 

 last-named, as in the case of the Ferns, the prothallia are developed vegetatively 

 from the spores of the large plant, and these again give rise sexually to an Equise- 

 tum, a Lycopodium, etc. In the Mquisetinae the spores are externally exactly alike, 

 but some give rise to male, others to female prothallia. In the case of the Sydrop- 

 terideae and the heterosporous Lycopodinae {Selaginellae, Isoeteae) the spores 

 from which the male prothallia are derived are smaller (microspores) but more 

 numerous than those which give rise to the female prothallia (macrospores). At 

 the same time, the prothallium does not in all cases grow out of the spores as an 

 independent plantlet, but remains within it and only exposes the sexual cells for 

 purposes of fertilisation ; so that the male sexual cells are produced within the micro- 

 spores and the egg-cells within the macrospores. Thus, in the higher Cryptogams 

 the alternating sexual generation, or the one producing the sexual cells, remains 

 concealed within the spores. In Phanerogams (Gymnosperms and Angiosperms) 

 the sexual generation has undergone even greater reduction. It has nevertheless 

 been determined that the pollen grains of the Phanerogams correspond to the 

 vegetatively produced microspores of the Vascular Cryptogams, and that in them 

 the male sexual cells also arise through a process of division. Similarly, the 

 embryo-sac of the Phanerogams, in which, in addition to the more or less reduced 

 prothallium (synergidse, antipodal cells), the female sexual cell (the egg-cell) occurs, 

 must be regarded as the equivalent of the asexually produced macrospores. The 

 young plant (the embryo), just as in Selaginella, is also formed in the macro- 

 spores — that is, in the embryo-sac. Viewed in this way, it is evident that an 



ALTERNATION OF GENERATIONS TAKES PLACE ALSO IN PHANEROGAMS. HoE- 



meister, the discoverer of this most important fact, drew most ingenious infer- 

 ences from it concerning the genetic connection of the higher with the lower plants, 

 of Phanerogams with the Vascular Cryptogams. 



In the alternating generations are clearly manifested the essential 

 functions of both modes of propagation — the quantitative, in the extra- 

 ordinary multiplication by asexual reproduction ; the qualitative, in the 

 sexual fusion. For while thousands of asexual spores are produced 

 from a single Fern-leaf, from the prothallium of the sexual generation 

 seldom more than one new Fern-plant arises, but that one plant derives 

 a qualitative value from the cross-fertilisation necessitated by the dicho- 

 gamy of the prothallia. , # 



Just as the Fern-plant can occasionally arise by budding (p. 279) 

 directly from the prothallium, without the intervention of a sexual act, 

 the formation of spores is also sometimes omitted, and the prothallia 



