MYXOPHYCOPHYTA 28I 



motion. The actual mechanism said to be responsible for loco- 

 motion is rhythmic longitudinal waves passing from end to end of 

 the trichomes and caused by changes in the volumes of the proto- 

 plasts. Yet another type of structure, thick- walled resting spores or 

 akinetes, occur in many of the filamentous forms belonging to the 

 Nostocaceae and Rivulariaceae, normally developing next to a 

 heterocyst, either singly or in a series. On germination they give rise 

 to a new filament. The entire lack of sexuality must be ascribed to 

 the ancient cell structure and the absence of chromosomes together, 

 possibly, with the lack of sterols. 



This type of cell structure naturally provides a problem for the 

 geneticist. It is probable that each cell contains a number of genes 

 which, if there are no chromosomes, must presumably be separated 

 from each other, and they v^ either be distributed generally 

 throughout the cell or else confined to a part of it. Since there is no 

 special means of accurate partition, sexuaUty would be useless be- 

 cause it could not confer any real property of recombination and 

 maintenance of gene number. 



Many of the forms aggregate into colonies, but in some of the 

 Chroococcaceae the macroscopic plant mass represents an associa- 

 tion of such colonies rather than a single colony. The form which 

 any colony may take up depends on (i) planes of cell division, (2) 

 effect of environment, which may determine the consistency of the 

 mucilage, uneven temperatures, for example, sometimes producing 

 irregular grov^h. It has been shown experimentally that the en- 

 vironment may affect the shape of colonies of Microcystis and 

 Chroococcus turgidus and determines the size of Rivularia haema- 

 tites (cf. p. 289). Certain fines of morphological development have 

 been followed by the group and may be depicted schematically as 

 on page 282 (cf. also Figs. 160-72). 



As may perhaps be expected from a primitive and ancient group 

 there is evidence of homoplastic or parallel development when 

 compared with plants from other primitive groups, especially the 

 Chlorophyceae. Homoplasy can be seen between Gloeothece and 

 Gloeocystis, Merismopedia and Prasiola (Figs. 30, 163), Chamae- 

 siphon and Characium (Figs. 16, 164), Chroococcus and Pleurococcus 

 (Figs. 39, 162), Lynghya and Hormidium, Stigonema and Witt- 



rockiella. 



As a group, the plants are extremely v^ddely distributed over the 

 face of the earth under all sorts of conditions, frequentiy occurring 



