EUPHYCOPHYTA 5I 



species vary considerably in size, shape and wall thickness. When 

 the walls are thick they are often lamellate. There is a single chloro- 

 plast which forms a characteristic circular band around the whole 

 or most of the cell circumference. Vegetative reproduction can take 

 place through fragmentation. 



Swarmers are formed from all cells of the filament except the 

 attachment cell, but they usually appear first in the apical cells and 

 then in successive cells below. They are liberated through a hole in 

 the side of the cell into a delicate vesicle, which subsequently bursts 

 and liberates the swarmers. Asexual reproduction is by means of 

 biflagellate zoospores, the numbers produced per cell depending on 

 their width: wide cells give 2, 4, 8, 16 or 32 zoospores per cell, 

 narrow cells i, 2 or 4 per cell. These zoospores are quadriflagellate 

 but in the larger species there are bi- or quadriflagellate microzoo- 

 spores which only germinate at low temperatures. Undischarged 

 zoospores turn into aplanospores and these may even germinate in 

 situ before liberation. 



Sexual reproduction is by means of biflagellate gametes of which 

 8, 16, 32 or 64 are produced per cell. The plants are dioecious with 

 plus and minus strains. No parthenogenetic development of gametes 

 has been recorded. The zygote germinates after 5-9 months giving 

 rise to 4-16 bodies which function either as aplanospores or as zoo- 

 spores. Since meiosis occurs at their formation the adult plant is 

 haploid. The plants appear mostly in winter or spring and opti- 

 mum conditions would seem to include low temperatures. The 

 genus is well represented in both fresh and salt waters. 



Genera closely allied to Ulothrix are Schizomeris, in which some 

 of the cells may divide longitudinally and in which, so far, sexual 

 reproduction has not been observed; Hormidium^ the filaments of 

 which have a strong tendency to fragment, and Binuclearia where 

 the cells are in pairs in a gelatinous filament. Hormidium flaccidum 

 is a widely distributed soil alga (see p. 380). 



MiCROSPORACEAE : Af zcro^pora {micro, small; spora, seed). Fig. 24 



This genus is sufficiently distinct from the preceding one to 

 warrant its inclusion in a separate family. The species are all fresh- 

 water and are free-floating, consisting of unbranched threads with 

 walls of varying thickness, the thicker walls showing stratification. 

 Each cell wall consists of two overlapping halves held in place by a 

 deUcate membrane. This structure arises from the fact that the 



