INTRODUCTION 19 



layer or cuticle, whilst in others there is an outer mucilage layer, 

 and in at least three groups (Siphonales, Siphonocladiales and 

 Charales) lime may be deposited on the walls. The chloroplasts 

 normally contain rounded bodies, or pyrenoids, which are composed 

 of a viscous mass of protein. The pyrenoids are usually surrounded 

 by a starch sheath, starch being the principal product of photo- 

 synthesis, and it is said that parts of the pyrenoid are successively 

 cut off to form starch grains, but the evidence for this is not entirely 

 satisfactory. The pyrenoids are perpetuated by simple division but 

 they may also arise de novo. 



Each cell usually contains one nucleus, but in certain groups a 

 multinucleate condition is to be found. Each nucleus possesses a 

 deeply staining body, the nucleolus, together with chromosomes 

 which are usually short and few in number, although these latter 

 may be masked during the interphases between nuclear division. 

 At cell division the pyrenoids and chloroplasts may also undergo 

 division. The flagellae of the motile bodies are composed of an 

 axial cytoplasmic filament surrounded, except at the very apex, by a 

 sheath which probably has the power of contraction, whilst in the 

 Volvocales the flagellae normally disappear at the commencement 

 of cell division. The motile cells also possess a red eye-spot, the 

 detailed structure of which is not yet elucidated in all the groups, 

 though it appears to contain a primitive lens in the Volvocales. The 

 red colouring matter is due in part to the chromolipoid pigment 

 known as haematochrome (cf. fig. 13). 



Vegetative reproduction takes place through fragmentation and 

 ordinary cell division, whilst asexual reproduction is by means of 

 bi- or quadriflagellate zoospores which are commonly produced in 

 normal cells because special sporangial structures are rare. These 

 zoospores are often formed during the night and are then liberated 

 in the morning : after liberation they may remain motile for as much 

 as 3 days or for as short a time as 3 min. Their production can some- 

 times be artificially induced by altering the environmental condi- 

 tions, e.g. removing the plant from flowing to still water {Ulothrix, 

 Oedogonium), changing the illumination, transferring to water from 

 air (terrestrial Vaucheria spp.), or removing from water for 24 hours 

 (Ulva, Enter omorpha). Each individual cell may produce one or 

 more zoospores, the number varying with the different species. 

 Liberation is secured by means of (a) lateral pores, {h) terminal 



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