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!; 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?. 
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 
pions. 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 

1 Bot. Ztg. 1877. ? See Pringsheim’s Jahrb f. wiss, Bot. I. 
