50 



CHLOROPHYCEAE 



The plants are dioecious in respect of sexual reproduction and several 

 of the species exhibit anisogamy. Biflagellate gametes from separate 

 plants fuse to give a non-motile zygote which then increases in size 

 and after some months undergoes meiosis and forms zoospores. 

 The macroscopic plants are thus all haploid and the diploid 

 generation is only represented by the enlarged zygote. In this 

 respect it is sharply differentiated from the genera Ulva and 

 Enter omorpha, and it possibly has only a distant relationship with 

 them. Each zoospore from the zygote divides to give eight peri- 

 pherally arranged cells with a central cavity and this then develops 

 slowly into a sac. The genus is more widespread than is perhaps 

 suspected from the literature, frequenting both saline and fresh 

 waters. 



*Ulvaceae: Ulva (Latin for a marsh plant). Figs. 36, 38. 



The thallus, which is composed of two layers and is therefore 

 distromatic, develops from a single uniseriate filament that sub- 



Fig. 38. Ulva lactuca. 

 Oltmanns.) 



A, plant. B, transverse section of thallus. (After 



sequently expands by lateral divisions, but there is usually no 

 hollow sac, though exceptions to this are found in U. Linza and 

 U. rhacodes. The plant is attached at first by a single cell, but later 



