122 A TEXTBOOK OF THEORETICAL BOTANY 



The zoospore may remain active for up to an hour and a half, after which 

 it becomes sluggish, the flagella are withdrawn, the cell becomes spherical 



and a wall is laid down. 



Division soon follows and a filament is formed. As growth continues the 

 cells at the posterior end begin to divide longitudinally so that a widening of 



Fig. 104. — Aglaozonia reptans. Vertical section of 

 thallus showing zoosporangia on upper surface. 



the filament occurs and it begins to assume a thalloid form, while rhizoids 

 develop from the lower end (Fig. 106). Gradually this lower portion, 

 developing by lateral growth, forms a funnel-shaped structure. From the 

 margin of this structure delicate multicellular filaments grow out, which 

 become the apical tufts of branches, and gradually the typical Ciitleria plant 

 becomes differentiated. 



Alternation of Generations 



Cutleria shows an alternation of generations between a large gameto- 

 phyte plant which is unisexual and a small sporophytic plant. In the 

 development of its reproductive organs it is essentially comparable with 

 Ectocarpus, though there is more differentiation both in the number and size 

 of the two kinds of gametes. It may be therefore regarded as a specialized 

 type derived from a form like Ectocarpus, in which the asexual plants have been 

 reduced in size, while the gametophyte plants have become more complex, 

 especially in their vegetative structure. In certain genera of the Ectocarpales 

 we have examples in which longitudinal division of some of the cells of the 

 filaments occur, and thus it is possible to see the way in which the thallus 

 of Cutleria may have originated. The zoospores produced as a result of 

 meiosis give rise to male and female plants, and it is reasonable to assume 

 that as a result of genetic segregation these male and female plants should 



