EUPHYCOPHYTA 



45 



sents optimum light conditions for growth of//, reticulatum, whilst 

 the actual growing period depends upon Hght intensity, e.g. it is 3 

 weeks at 600-700 lux, 5J weeks at 250 lux. Under normal condi- 

 tions growth is periodic, increasing by day and decreasing at night. 

 This is reflected in the oxygen balance between photosynthesis and 



Fig. 20 Hydrodictyon. A-F, development of young net of H. 

 patenaeforme from the zygote. A, young polyhedron. B, older 

 polyhedron with four nuclei. C, protoplasm granular just before 

 zoospore formation. D, 'pavement' stage. E, zoospores rounding 

 off and wall of polyhedron expanding to form vesicle. F, fully 

 formed net still enclosed in vesicle. (A-E x 250, F x 175.) G, 

 portion of mature net of H. reticulatum. H, polyhedron and young 

 net of H. reticulatum. J, H. reticulatum, formation of net in parent 

 cell from zoospores. (A-F, after Pocock; G, H, after Oltmanns; J, 



after Fritsch.) 



respiration. Altering the length of the light period merely alters the 

 degree of growth in the time. Under conditions of continuous Hght 

 the periodicity is much reduced, though the effect is more marked 

 with old cells. Light is essential for growth, as in continuous 

 darkness the oxygen balance falls below the compensation point. 



