EUPHYCOPHYTA 



55 



scopic plants are thus all haploid and the diploid is only repre- 

 sented by the enlarged zygote. In this respect it is sharply differ- 

 entiated from the genera Ulva and Enter omorpha (see below). 

 Each zoospore from the zygote divides to give eight peripherally 

 arranged cells with a central cavity and this then develops slowly 

 into a sac. The genus is more widespread than is perhaps thought, 

 frequenting both saHne and fresh waters. 



* Ulvaceae: Ulva (latin for a marsh plant). Figs. 26, 28 



The thallus, which is composed of two layers, develops from a 

 single uniseriate filament that subsequently expands by lateral 

 divisions, but there is no hollow sac, though in U. linza and U. 

 rhacodes the two layers may separate towards the margins or near 



nOsBe^K 



Fig. 28 Ulva sorensenii. A, plant. B, cells in surface view. 

 Cj transverse section of thallus. (After Chapman.) 



the base. The plants are attached at first by a single cell, but later 

 multinucleate rhizoids grow down from the lower cells and form a 

 basal attachment disc that may persist throughout the winter, new 

 plants arising from it in the spring. Detached fragments are an- 

 other means of forming new thalli especially in stagnant-water 

 forms, whilst normal asexual reproduction is by means of quadri- 

 flagellate zoospores. 



