EUPHYCOPHYTA 19 



there is a ring of cilia, which are commonly produced in cells that 

 do not differ from ordinary vegetative cells. These zoospores are 

 often formed during the night and are then liberated in the morn- 

 ing: after liberation they may remain motile for as much as three 

 days or for as short a time as three minutes. Their production can 

 sometimes be induced artificially by altering the environmental 

 conditions, e.g. removing the plant from flowing to still water 

 {Ulothrix, Oedogonium), changing the illumination, or removing 

 from water for twenty-four hours {Ulva, Enter omorphd). Each 

 individual cell may produce one or more zoospores, the number 

 varying with the different species. Liberation is secured by one of 

 the following means : (a) lateral pores, {h) terminal pores, (c) gela- 

 tinization of the entire wall, {d) the wall dividing into two equal or 

 imequal portions. 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. Aplanospores 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. i). 



Sexual reproduction is represented in all the orders and often 

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

 times being retained and fertilized on the parent thallus in the 

 oogamous forms (e.g. Volvox^ Oedogonium, Coleochaete, Charales). 

 The isogamous forms are normally dioecious, the two sexes being 

 termed + and - , because as plants and gametes are aUke morpho- 

 logically they can only be distinguished by the fact that the gametes 

 can normally only fuse with gametes derived from another plant. 

 In some cases (Ulva, Enteromorpha, Chlamydomonas) relative 

 sexuality is known to occur, weak + or - strains fusing with 

 strong + or - strains respectively (cf. p. 417). Segregation into + 

 and - strains occurs during meiosis, a phenomenon which in many 

 species takes place at the first division of the fertilized egg or zygote. 



The isogametes and also the zoospores can be regarded as a 

 temporary reversion to the primitive flagellate condition. In the 

 case of algae with oogamy the gametangia (antheridia (male) and 

 oogonia) are of distinctive shape. In some genera gametes are 

 capable of developing parthenogenetically into new plants. 



The occurrence of sexual reproduction in nature often marks 

 the phase of maximum abundance when the climax of vegetative 



