20 



CHLOROPHYCEAE 



pores, (c) gelatinization of the entire wall, (d) the wall dividing into 

 two equal or unequal halves. In some species non-motile zoospores 

 are formed which are called aplanospores, but if these should then 

 secrete a thick wall they become known as hypnospores. Aplano- 

 spores which have the same shape as the parent cell are termed 

 autospores. All these spores develop a new membrane when they 

 are formed and hence differ from a purely resting vegetative cell or 

 akinete (cf. fig. 13). 



B A 



Fig. 13. A, diagram of eye-spot of C/zZaw3'^owonas. _^ = pigment cup, 5 = photo- 

 synthetic substance. B, diagram of cross-section of eye-spot of Volvox. L = lens, 

 /)'= pigment cup, 5 = photosynthetic substance. C, aplanospores of Microspora 

 Willeana ( x 600). D, akinete of Pithophora oedogonia ( x 225). (After Smith.) 



Sexual reproduction is represented in all the orders and often 

 there is a complete range from isogamy to oogamy, the ova usually 

 being retained on the parent thallus in the oogamous forms (e.g. 

 Vaucheria, Coleochaete). The isogamous forms are normally di- 

 oecious, the two strains being termed + and - , and as they are 

 usually alike morphologically they can only be distinguished by the 

 behaviour of the gametes. In some cases ( Ulva) relative sexuaUty is 

 known to occur, weak + or - strains fusing with strong + or - 

 strains respectively. Indeed, Hartmann (1924) has declared that 

 all gametes are potentially bisexual, and there would seem to be 

 considerable grounds for supporting this view. Segregation into + 

 and - strains occurs during meiosis, a phenomenon which in 

 many species takes place at the first or second division of the zygote. 



