XV NATURE OF THE ALTERNATION OF GENERATIONS 563 



preliminary formation of zoospores, gives rise to the new gen- 

 eration. The primary function of the resting spore (zygote) 

 is to carry the plant over a period of stress — drought or cold. 



The Confervoidese among the Green Algae are for good 

 reasons considered to be among living forms the nearest to the 

 progenitors of the Archegoniates. The germinating zygote in 

 these plants usually develops several zoospores, each of which 

 gives rise to a new plant, thus quickly increasing the number 

 of individuals resulting from a single fertilisation. This is 

 obviously an advance upon the condition where the zygote gives 

 rise to but one plant, and this preliminary division of the zygote 

 probably was the first step in the evolution of the sporophyte or 

 neutral generation which becomes so conspicuous in the Arche- 

 goniates. 



Among the Confervoidese, Coleochccte most nearly approxi- 

 mates the condition found in the lower Bryophytes. Alone 

 among the Algae the germinating zygote forms a cellular body 

 or embryo directly comparable to that of Riccia, for example. 

 Each cell of this embryo-sporophyte then produces a zoospore 

 which develops into a new plant (gametophyte). 



Whether the protective envelope formed about the fertilised 

 oogonium of Coleochccte may be considered to be in any way 

 comparable to the outer cells of an archegonium is doubtful — 

 at best the resemblance is very remote — and in the character of 

 the sexual organs there is a very great gap between Coleochccte 

 and the simplest Liverwort. 



The zygote of the Green Algae is evidently a provision for 

 carrying the plant over periods of cold and especially drought — 

 that is, it is in a sense an adaptation to terrestrial conditions 

 which the growing plant cannot withstand. From this dormant 

 unicellular sporophyte (oospore) there has gradually been 

 evolved the complex, independent sporophyte of the vascular 

 plants. 



The first step in the elaboration of the sporophyte w^as the 

 production of several zoospores. The next step is that shown 

 in Coleochccte, where there is marked growth of the germinat- 

 ing zygote and its transformation into a cellular body, or 

 embryo, previous to the formation of the zoospores. No form 

 is known among the Chlorophyceae in which the development of 

 the sporophyte is carried any further. 



The transition from the typically aquatic life of the algal 



