ALTERNATION OF GENERATIONS. 20^ 



of plants, but also to compare the process according to definite principles, as 

 will be done in detail in the Second Book. Here it is necessary only to remark 

 that alternation of generations occurs even in Phanerogams ; the Cycadeee and 

 Coniferse are in this respect nearly related to Lycopodiacese, and through them 

 we are also able to trace a repetition of the most important features of the alter- 

 nation of generation of the highest Cryptogams in the formation of the seeds of 

 Monocotyledons and Dicotyledons. It is sufficient only to remark that the Endo- 

 sperm of Phanerogams corresponds to the Prothallium of Vascular Cryptogams 

 (and thus to the sexual generation), while the Embryo which lies by the side 

 of or enclosed in the endosperm may be compared to the asexual spore-forming 

 generation of Ferns, Equisetacese, &c. In Phanerogams, as in Vascular Cryptogams, 

 one generation (the endosperm or prothallium) is morphologically a thallus, the 

 other a cormophyte, the rooting stem with its leaves and flowers. 



Since the principal divisions of the life-history of each plant, and the decisive 

 turning-points of the different formative processes, are given in the alternation of 

 generations, a systematic representation of the natural relationship of plants — in 

 other words the natural system — should primarily demonstrate and compare the 

 homologies in the alternation of generations in different classes. This at the 

 present time is attended with great difficulties in Thallophytes and Characeae, and 

 indeed is to a certain extent still impossible; but in Muscineae, Vascular Crypto- 

 gams, and Phanerogams, it is practicable, and leads to the most unexpected and 

 interesting results. 



In the corresponding paragraph in the first edition I placed at the foundation of the 

 definition of alternation of generations the phenomena which occur in Miiscineae and 

 Vascular Cryptogams, and, to correspond with this, brought into prominence as the 

 essential condition the change of the law of growth in the passage from one alternate 

 generation to the other ; this is true for many Thallophytes on the one hand, and 

 for Phanerogams on the other. In this view, however, the intimate relation of the 

 production of sexual and asexual reproductive cells to the alternation of generations 

 was kept too much in the background, since alternation of generations in its simplest 

 forms, as in Vaucheria, Saprolegnia, and the Mucorini, is simply almost invariably 

 an alternation of sexual and asexual generations otherwise scarcely differing in their 

 mode of growth. Although the earher definition might, in its strict sense, be applied 

 to these cases also, the representation attempted here appears to me clearer and more 

 easily intelligible to beginners. The earlier definition permitted also the production 

 of the Moss-buds on branches of the protonema to be comprised under the head of 

 alternation of generations, — a case which, on the other hand, cannot be included under 

 the definition here given, since the apical cell of a lateral branch of the protonema, 

 which becomes transformed into the apical cell of the leaf-shoot, cannot be considered, 

 without further explanation, as a reproductive cell in the above sense. But if the defi- 

 nition of alternation of generations should seem to be injured by the exclusion from it of 

 this remarkable phenomenon, it must be specially noted, on the other hand, that the 

 sharp differentiation of the I\Ioss-buds from the protonema which produces them, and 

 the passage to an entirely different law of growth, does not occur in the Hepaticse. 

 The structures in them homologous to the protonema frequently appear only as a 

 preparatory and transitory germ-condition, just as germinating Fern-spores, especially 

 those of Hymenophyllaceae, first produce a filiform structure, at the apex of which 

 is formed the flat prothallium. The protonema may therefore be considered, with 

 the so-called Pro-embryo, as a peculiar process of growth interpolated in the alter- 



