DEVELOPMENT OF PLANTS 



193 



develop into feeble plants. This weakness is overcome by the 

 fusion of two gametes which results in the formation of a cell 

 or gametospore with renewed energy for growth. We also note 

 how these departures in the behavior of the organism are of 

 advantage to it. The zocspores effect a distribution of the 

 species while the gametospore ensures a continuity of the species 

 by tiding it over unfavorable conditions. 



The gametospore germinates after a period of rest somewhat 

 as in the case of Spirogyra. The contents of the spore, instead 

 of giving rise directly to a new plant, divides, forming several 

 zoospores (Fig. 112, E, F) which after a period of motility 



FIG. 112. Sexual reproduction of Ulothrix: A,, a. few cells of a filament 

 showing the formation and escape of the gametes. B, gamete. C, D, stages 

 in the union of the gametes. E, gametospore. F, gametospore germin- 

 ating and forming the sporophyte or asexual generation, which is a sac-like 

 plant containing several zoospores. 



become attached to some object and grow into a new plant. 

 The cause of this peculiar behavior of the germinating gameto- 

 spore is not known. Some change has occurred in it that 

 renders it incapable of developing directly into a new plant. 

 This variation in the growth of the gametospore marks the 

 beginning of one of the most important departures in the evo- 

 lution of plant life and it will become more and more significant 

 as the work proceeds. It is evident that the formation of 



