THE ALGAE 8i 



a broad transverse band lying close against the wall and curved round the 

 cell in the form of a ring. Situated in the chloroplast are several pyrenoids. 

 The centre of the cell consists of a large vacuole containing cell sap. 



In cases where the filaments are attached at the end, the anchoring cells 

 are devoid of chlorophyll and are either brown or nearly white in colour. 

 There is no special growing region and any cell of the filament may divide 

 as elongation takes place. Vegetative multiplication is often effected by 

 fragmentation of the filaments. 



Asexual Reproduction 



This consists in the formation of zoospores. They arise from the 

 ordinary cells of the filament (Fig. 62, b), which divide up into a number of 

 separate parts, generally from two to sixteen, though occasionally only a single 

 zoospore may be produced from the mother cell. They are liberated into a 

 delicate mucilage vesicle through a small aperture on one side of the cell. 



The zoospores are of two kinds, macrozoospores, which are slightly 

 flattened and have four flagella, and microzoospores, which are ovoid and 

 have either four or two flagella. The latter type form an intermediate group 

 between the macrozoospores and the gametes, and are not clearly distinguish- 

 able from the latter in appearance. 



Each zoospore contains a single nucleus ; the chloroplast is situated in the 

 broader part of the spore, while the apical end consists of clear protoplasm, 

 attached to which are the flagella. There is a single contractile vacuole 

 which pulsates about every ten seconds. Each zoospore is provided with 

 an eye spot which is apparently sensitive to light. In light of moderate 

 intensity they swim towards the source of light, but in very brilliant illumina- 

 tion they swim in the opposite direction. Finally, after swimming about for 

 several hours, they come to rest, attach themselves and grow out directly 

 into new filaments. On germination the first cell produced is a colourless 

 rhizoidal cell attached to the substratum. 



Sexual Reproduction 



The gametes are produced either in cells of the same filaments as the 

 zoospores or in different ones (Fig. 63). They resemble the microzoospores 

 closely, except that each possesses only two flagella. Each mother cell 

 divides to produce from eight to thirty-two gametes. Conjugation normally 

 takes place only between gametes from different filaments. Fusion is similar 

 to that described in Chlamydomonas, forming a zygote with four flagella. 

 After fusion movement does not continue for long and soon the flagella are 

 retracted and the zygote comes to rest, develops a thick wall and is trans- 

 formed into a zygospore. It divides into a number of non-flagellated cells 

 (aplanospores) which are liberated and form new filaments. The first 

 division of the zygote is the reduction division, so that the cells of the filaments 

 are monoploid. 



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