30 



PLANT MORPHOLOGY 



sperms hang together as a plate, but finally separate and fuse with the 

 eggs. The union of a sperm and egg, called fertilization, results in 

 the formation of a thick-walled resting zygote. Upon germination, 

 the zygote produces four biciliate zoospores but only one functions, the 

 other three degenerating inside the zygote. Because the pairing gametes 

 are differentiated into sperms and eggs and are therefore unlike, Eudorina 



Fig. 19. Eudorina elcgans. A, colonj' of 32 cells, many of which are dividing to form 

 daughter colonies, X 500; B, a female colony surrounded by numerous sperms, two groups of 

 which are still intact, while others, having separated, are uniting with the eggs. (After 

 Gocbel.) 



is heterogamous. Some species show a further degree of sexual differentia- 

 tion in being dioecious. Here all the cells in the male colony give rise 

 to sperms, while all those in the female colony become eggs. 



Volvox. This is the most highly developed member of the Volvocales. 

 It lives in quiet bodies of fresh water, especially pools, ponds, and lakes. 

 It consists of a hollow globular colony composed of hundreds or some- 

 times thousands of biciliate cells embedded in mucilage and arranged in 

 a single layer (Fig. 20A). Often the colony reaches a diameter of 

 nearly 2 mm. It is free-swimming, as in other members of the order. 

 Each cell is like an adult Chlamijdomonas, with two cilia, an eyespot, 

 contractile vacuoles, a nucleus, and a single chloroplast with a pyrenoid. 

 In most species the cells are connected by very fine protoplasmic strands, 

 and thus the colony approaches the multicellular condition of organiza- 

 tion. This is also shown by the fact that most of the cells function only 

 vegetatively during the entire life of the colony, while others become 



