ECTOCARPUS 



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 algse. 



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- 

 rangium. 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- 

 ture, the sexually formed cell is 



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 ; B, the digitate type 

 (Laminaria digitata), which is never 

 very long, hut is broad at the base 



