ECTOCARIM'S 



207 



sporangia, to distinguish them from the sexual organs, but the 

 structure is clearly the same as that of the one-celled sporangium 

 of the green algie. 



The sexual organs are devel- 

 oped from side branches, most 

 of whose cells divide repeatedly 

 until an immense number of 

 small compartments are formed. 

 The filament thus becomes 

 transformed into a complicated 

 many-celled organ (Fig. 193, B) 

 made up almost wholly of small 

 cubical cells, each of which de- 

 velops a single two-ciliate ga- 

 mete similar to a zoospore, or, 

 perhaps, two or three of these 

 motile elements. Because the 

 gametes are developed in small 

 compartments, the organ has 

 been termed a plurilocular spo- 

 ranyium. It is clear that this 

 many-celled organ is a very dif- 

 ferent sort of structure from the 

 one-celled reproductive organs. 

 It marks an important advance 

 in the evolution of reproductive 

 structures in plants and suggests 

 the many-celled sexual organs 

 characteristic of the bryophytes 

 and pteridophytes. 



The gametes are known to 

 fuse in pairs (Fig. 193, C), as in 

 many simple green algae, and 

 since they have a similar struc- 



(Laminaria digilata), which is never 



ture, the sexually formed cell is very i ong , but is broad at the base 



FIG. 194. Kelps from the North 

 Atlantic 



A, the simple type of Laminaria, some 

 of whose species grow to be thirty or 

 more feet long: />, the digitate type 



