ALTERNATION OF GENERATIONS 195 



The fungi and algas are omitted from these remarks. In the 

 former there is nothing analogous to the sporophyte and the gameto- 

 phyte. In algae like spirogyra, evidently the whole plant is a ga- 

 metophyte and, since the zygospore germinates directly into a new 

 gametophyte, there is probably no sporophyte. In some other algse 

 traces of a sporophyte have been found, but the discussion of these 

 would lead too far for the present purpose. 



In the ferns the egg-cells are developed on the prothallus. 

 This then is the gametophyte. It corresponds to the thallus of mar- 

 chantia and to the "moss plant," but it has become much reduced. 

 The plant developing from the fertilized egg-cell is the large and 

 beautiful " fern plant" differentiated into stems and leaves. Since the 

 fern plant produces the spores directly, it is the sporophyte and 

 corresponds to the shaft and capsule of the mosses. Both sporophyte 

 and gametophyte lead an independent existence. 



As we pass on to equisetum and isoetes, the sporophyte is still 

 more conspicuous in comparison with the gametophyte. In isoetes the 

 prothallus (gametophyte) is very rudimentary, consisting only of a 

 few cells remaining within the spore, which merely bursts to expose 

 the archegonia or to allow the sperm-cells to escape. Moreover, the 

 spores have become differentiated into micro- and macrospores corre- 

 sponding to the pollen and embryo-sac of higher plants. 



This gradual increase of the sporophyte and reduction of the 

 gametophyte can be traced on through the flowering plants in which 

 "the plant" is the sporophyte, and the gametophyte is represented 

 simply by a few cells in the germinating pollen grain, and in the 

 embryo -sac. 



One of the tuft-mosses (Leucobryum). 



Outside and inside views of a tuft, the latter showing the radiating 

 stems extending to the light. 



