THE ORIGIN OF SEX IN PLANTS 



187 



four-ciliate asexual zoospores by its two pigment spots. It finally 

 comes to rest and remains quiet for many weeks or several 

 months, but slowly increases in size. Finally, the zygospore 

 develops several zoospores (Fig. 180, G), which escape from the 

 cell, swim off, and develop new Ulotlirix plants. 



218. The sea lettuces. The sea lettuces include Ulva and 

 its relatives (family Ulvacece), and are very common along the 

 seacoast, forming green 



fringes on the rocks and 

 wharves near low water- 

 mark. The thallus is a 

 thin green membrane (Fig. 

 181,^4) instead of a fila- 

 ment as in Ulotlirix. Zoo- 

 spores and gametes (Fig. 

 181, B, C) are developed in 

 the cells along the edge 

 of the membranes. Their 

 structure, methods of 

 formation, and habits are 

 essentially the same as in 

 Ulothrix, and the sea let- 

 tuces are equally good for 

 the study of these points, 

 and they are sometimes 

 more available than Ulo- 

 tlirix for those living at or 

 near the seacoast. 



219. The origin of sex in plants. Ulotlirix, Ulva, and some 

 other types show clearly that the simplest forms of gametes 

 in plants are closely related to zoospores, for they are devel- 

 oped in the same way and have a similar structure. Indeed, 

 the gametes of these lower plants frequently germinate directly 

 like zoospores, thus showing that the sexual habit of fusing 

 with one another is not firmly fixed. However, the plants that 



FIG. 181. The sea lettuce (Ulva) 



A, habit sketch; B, cells forming four-ciliate 

 zoospores ; C, two-ciliate gametes. Adapted 

 after Thuret 



