46 CHLOROPHYCEAE 



then a rhizoid and filament grow out opposite each other in a plane 

 at right angles to the longitudinal axis of the original zoospore. 

 (b) Each cell produces four, eight, sixteen or thirty-two bi- or quadri- 

 flagellate microzoospores. These swarmers will only germinate at 

 low^ temperatures and then more slowly than the macrozoospores, 

 producing a somewhat narrow filament or else forming resting 

 spores, (c) Eight, sixteen, thirty-two or sixty-four biflagellate 

 gametes are produced in each cell and are usually liberated soon 

 after daybreak. The adult plants are usually dioecious, and after 

 fusion of the gametes has taken place (parthenogenesis is said not to 

 occur) the quadriflagellate zygote forms a resting zygospore. This 

 germinates after 5-9 months giving rise to four or sixteen aplano- 

 spores, and as meiosis occurs during their production the adult 

 plants are haploid. 



It is said that there are six types of adult filament: + and — 

 strains producing + or — gametes only, + and — strains producing 

 both + or — gametes and zoospores and + or — strains producing 

 zoospores only. Aplanospores, when they are formed, may either 

 develop into new plants or else they form a temporary "palmella" 

 state. Akinetes are also recorded. The genus appears primarily in 

 winter or spring, and the optimum conditions would seem to include 

 either cold weather or cold water because the plants die down in 

 summer. The genus is well represented in both fresh and salt waters, 

 U. zonata and U. flacca being common species respectively of the 

 two habitats. The nearly related genus Schizomeris, in which the 

 filaments have some longitudinal divisions, may be considered as 

 representing an intermediate stage in the evolution of the more 

 foliaceous forms, e.g. Ulva. 



*MiCROSPORACEAE : MicrospoYU {micro, small ; spora, seed). Fig. 34. 



This genus is sufficiently distinct from the preceding one to 

 warrant its inclusion in a separate family. The plants are free- 

 floating when mature and consist of unbranched threads, the cells 

 of which have walls of varying thickness, the thicker walls showing 

 some stratification. In many species the cell wall is in two over- 

 lapping halves held in place by a delicate inner or outer membrane, 

 and it is because of this type of structure that the threads readily 

 fragment into H pieces. In ordinary cell division growth is brought 

 about by the introduction of new H pieces. The parietal chloroplast 



