THALLOPHYTA: ALGAE 



61 



creeping axis with root-like holdfasts and erect leaf-like shoots of various 

 form. In some species the shoots reach a height of 30 cm. Cross walls 

 are absent in the vegetative part of the plant, but the central cavity is 

 traversed by numerous slender strands. Asexual reproduction occurs 

 only by fragmentation, sexual reproduction by biciliate isogametes. 



Summary. The Siphonales are characterized by a completely coeno- 

 cytic plant body that is usually much branched and often differentiated 

 in form. Cross walls appear only in connection with the formation of 



ABC 



Fig. 50. Three species of Caiilerpa, a coenocyte with a high degree of structural differentia- 

 tion, one-half natural size. A, Canlerpa prolifera; B, Caulerpa crassifolia; C, Caiilerpa 

 macrodisca. 



r?productive organs. The vegetative body contains innumerable nuclei 

 and small chloroplasts. It is really a single multinucleate cell. Asexual 

 reproduction may be accomplished by fragmentation of the thallus, by 

 zoospores, aplanospores, or akinetes. Sexual reproduction ranges from 

 isogamy to heterogamy. This is a highly specialized order related both 

 to the Chlorococcales and the Siphonocladiales. 



Summary of Chlorophyceae 



The Chlorophyceae are algae with only chlorophyll and its associated 

 carotinoids in their plastids, these being present in the same proportions 

 as in the higher plants. In vegetative organization they are highly 

 diversified. Some are unicellular but most of them are multicellular, the 

 thallus being most commonly filamentous, sometimes plate-like, and 

 rarely massive. Some are partially or completely coenocytic. There is 

 relatively little cellular differentiation. A definite cell wall composed of 

 cellulose is nearly always present, this seldom becoming mucilaginous. 

 The cells contain a well-organized nucleus (often more than one) and one 

 or more distinct plastids. Pyrenoids are usually present. Reserve food 

 is stored generally as starch, sometimes as oil. Asexual reproduction 



