iqh! COULTER— REPRODUCTION IN PLANTS 343 



duced under conditions unfavorable to vegetative activity, and 

 therefore unfavorable for the production of a new individual. The 

 conditions that favor zygote-formation inhibit zygote-activity, 

 and it responds with its heavy wall and dormant protoplast. Even 

 when this zygote germinates, it may not produce a new individual, 

 but the protoplast may divide at once to form spores. In other 

 words, the protoplast of a zygote may function directly as a spore- 

 forming protoplast. The inference is that the production of a 

 vegetatively active individual or the production of spores depends 

 upon the conditions for vegetative activity. If these conditions 

 favor maximum vegetative activity, a vegetative individual will be 

 produced; but if they do not favor maximum vegetative activity, 

 spores will be produced. The succession of conditions at the open- 

 ing of a growing season is just the reverse of the succession at the 

 ending of a growing season. In the latter case there is a gradually 

 waning activity, resulting in spore-formation following great vege- 

 tative activity; while at the opening of a season there is gradually 

 increasing activity, the conditions first favoring spore-formation, 

 and then vegetative activity. 



The seasonal relation between spores and gametes lies at the 

 basis of the changes which gradually established a definite alterna- 

 tion of generations. Spores are always as essential a feature of the 

 life history as gametes, and in plants sexual reproduction is never 

 the whole of reproduction. 



A summary of the important facts in reference to the "origin 

 of sex" may be stated as follows: (i) gametes have been derived 

 from zoospores which have become so reduced by succcessive divi- 

 sions as to be incapable of functioning as spores; (2) when first 

 recognized by their behavior, gametes are alike in every visible 

 feature, so that there is no evident distinction of sex; (3) a physio- 

 logical differentiation of gametes is indicated by their 



mu 



attraction in pairing, so that two sexes are present, although not 

 distinguishable; (4) gametes are formed under conditions relatively 

 unfavorable to either vegetative activity or spore-formation, repre- 

 senting the closing activity of a plant; (5) gametes therefore appear 

 in response to unfavorable conditions that arise in the life history 

 of a plant which is long enough to extend over a considerable 



