i9H] COULTER— REPRODUCTION IN PLANTS 341 



between plants without and with distinct sporangia is that in the 

 former all protoplasts may respond to the conditions for spore- 

 formation, while in the latter only a restricted number of proto- 

 plasts respond. What determines the selection is a question that 

 remains for physiology to answer. When the differentiation of 

 sporangia first takes place, the vegetative cells have either lost the 

 power of spore-formation once common to all vegetative cells, or 

 are inhibited from expressing it. Spore-formation by cells ordi- 

 narily vegetative occurs often enough to assure us that spore- 

 formation is only inhibited in such vegetative cells. 



A summary of what may be called three stages in the history 

 of asexual reproduction, as given above, may be stated as follows. 

 The first stage is represented by cell-division, which belongs to 

 cell-activity in general; in other words, it is a process as natural 

 .to all protoplasts as any work. The second stage is represented 

 by spore-formation, in which ordinary vegetative cells under certain 

 conditions produce spores. In this case the activities of a cell are 

 differentiated by varying conditions, and are not differentiated 

 permanently. The third stage is represented by the reproduction 

 of spores by special cells which are differentiated in function per- 

 manently from the ordinary vegetative cells. This specialization 

 of certain cells is accompanied by the inhibition of the spore- 

 producing power of cells in general. 



The origin of sex 



If all plants were sexual, the origin of sex would be as obscure 

 a problem as is the origin of life. Fortunately for this problem, the 

 most primitive plants are sexless, and the gametes are seen to be as 

 definitely related to previous structures as are any other features 

 of evolutionary progress. What is ordinarily referred to as the 



simply the morphological origin 



gametes, the visible 



made 



illustration in this connection is famili 



botanists. The gradations from zoospores to gametes are com- 

 plete, so that it seems to be clear that in this case gametes are 

 morphologically spores greatly reduced in size, and usually inca- 

 pable of functioning as spores. It is a temptation to infer that 



