CHAPTER IV. INTRODUCTION. 125 



careful examination and a comparison of allied species. In dealing with such plants 

 the observer is confronted by difficulties arising from the strong preponderance of the 

 propagative forms, from the variety of shapes which they assume in the same species, 

 a variety sometimes due to external causes, sometimes established by inheritance, and 

 lastly from the alternation in some species, partly due to external causes, partly also 

 hereditary and constant, between purely propagative and fructificative generations of 

 bions returning again to the typical rhythm. Excellent examples are afforded by the 

 history of the development of Botrydium and Acetabularia 1 ; many of the Oedogonieae 

 are connected with Botrydium as presenting a simpler case of what may be termed 

 facultative alternation of bions, while the gynandrosporous Oedogonieae in which this 

 alternation is a necessary part of their course of development connect with Acetabularia 2 . 

 In these instances we have succeeded in ascertaining the typical rhythm from the 

 successive production and reproduction of the forms. In many of the lower species 

 this has not yet been done, and there are still some comparatively highly differentiated 

 species in which the rhythm of the development is not yet fully made out ; this is 

 especially the case with the numerous species of the Florideae, in which the 

 propagative organs (tetraspores) are assigned to special plants having no archicarps 

 or antheridia. We have not yet exactly ascertained the genetic relation or possible 

 relation of alternation between these forms and those first mentioned; but we know 

 that the organs of propagation do not occur on special plants in all the Florideae, and 

 that they are not generally necessary members of the course of development in these 

 plants, but are entirely wanting in some species. 



Where fructificative and purely propagative generations of bions proceed 

 alternately from one another, it is also quite natural to speak of alternating generations 

 and of alternation of generations, using the latter expression therefore in a less limited 

 sense than Sachs. The practice is adopted in this work in accordance with the 

 original meaning of the word ; alternation of generations here indicates every kind of 

 course of development, which is made up of alternating generations of independent 

 bions. In this very obvious sense the word is both necessary and convenient. 

 In every other sense it means nothing more than the rhythm of the development, 

 and the homologous alternating generations are merely homologous stages of the 

 development. To term these stages generations, even where they are not independent 

 bions, may be convenient for comparing them with homologous independent bions ; 

 but if the practice is carried beyond certain narrow limits, into the succession of shoots 

 &c., every course of development necessarily becomes an alternation of generations, 

 and the expression ceasing to have a special signification becomes therefore 

 superfluous or objectionable. 



It has already been pointed out that the main series of the vegetable kingdom 

 contains species, in which the propagative form of development greatly preponderates, 

 and is in fact the only one which does or can make its appearance through a long 

 succession of generations. In these species therefore a fructification is a comparatively 

 rare occurrence. But the existence of the species is not prejudiced by this 

 morphological defect, for so it may be termed ; and experience shows further that the 

 preponderance of the propagative form of development may be carried so far, that no 



Bot. Ztg. 1877. 2 See Pringsheim's Jahrb f. wiss. Bot. I. 



