44 



CHLOROPHYCEAE 



shape from the others and often have processes which are elabora- 

 tions of the mucilaginous cell envelope : these processes are prob- 

 ably to be correlated with the planktonic mode of life, whilst tufts of 

 bristles performing the same function and similar to those of 

 Pediastrurn are also recorded. 



It should be evident from the preceding descriptions that the 

 Chlorococcales represent a number of very diverse types, some of 

 which may have indications of distant relationships whilst there 

 are others whose relationships are extremely vague : a recent paper 

 even describes some oogamous members. 



REFERENCES 



Chlorococcum. Bold, H. C. (i 930-1). Bull. Torrey Bot. Club, 57, 577- 

 Chlorochytrium. Bristol, B. M. (iqiq)- J- Linn. Soc. (Bot.) 45, i. 

 Chlorella. Grintzesco, J. (1903). Rev. Gen. Bot. 15, 5. 

 Pediastrurn. Harper, R. A. (1918). Proc. Amer. Phil. Soc. 57, 375. 

 Hydrodictyon. Mainx, F. (193 i). Arch. Protistenk. 75, 502. 

 Chlorochytrium. Palm, B. T. (1932). Rev. Alg. 6, 337. 

 Hydrodictyon. PococK, M. A. (i937)- Trajis. Roy. Soc. S. Afr. 24, 263. 

 Chlorococcum. Puymaly, A. de (1924). Rev. Alg. i, 107. 

 Scenedesmus. Smith, G. M. (19 14). Arch. Protistenk. 32, 278. 



ULOTRICHALES 

 *UxoTRiCHACEAE : Ulothrix {iilo, shaggy; thrix, hair). Fig. 33. 



The unbranched filaments are attached to the substrate by means 

 of a modified basal cell which frequently lacks chlorophyll, but even 

 though attached at first the plants sometimes become free-floating. 

 Under unfavourable conditions, e.g. nutrient deficiency, rhizoids 

 may grow out from the cells or else the filaments become branched. 

 This behaviour suggests one way at least in which the branched 

 habit may have evolved from the simple filament, in this case 

 probably representing an attempt to increase the absorbing surface 

 in order to counteract the deficiency of salts. The cells vary con- 

 siderably in size and shape and the walls may be thick or thin ; if the 

 former, then they are usually lamellate. There is a single chloro- 

 plast which forms a characteristic circular band around the whole or 

 most of the cell circumference. Vegetative reproduction can take 

 place through fragmentation, especially when conditions are un- 

 favourable, the various fragments developing conspicuous rhizoids. 

 Swarmers are formed from all the cells of the filament except the 

 attachment cell, but they usually appear first at the apex of the 



