2] THE VARIOUS GROUPS OF PLANTS 29 



filaments, or of colonies in which either individual cells or filaments 

 are held together in gelatinous masses. Examples, variously 

 magnified, are shown in Fig. 4 ; but whereas the individual cells 

 are microscopic and often exceedingly minute, the colonies may be 

 manv centimetres in diameter and of considerable bulk. 



The Cyanophyceae contain chlorophyll of a sort and mostly live 

 by photosynthesis although some appear to be at least partly sapro- 

 phytic. The chlorophyll is diffused through the outer layers of 

 protoplasm instead of being accumulated in special bodies as it is 

 in higher plants. Cyanophyceae also contain bluish and/or reddish 

 pigments, the former of which, with the chlorophyll, gives to many 

 a blue-green coloration — hence their popular name. Others, how- 

 ever, are very differently coloured. Multiplication is by simple cell 

 division or the formation of spores. The filaments or parts of 

 filaments of many exhibit motions of various but characteristic kinds, 

 including glidings and oscillations, the mechanisms of which are 

 not understood. 



Cyanophyceae are very widespread, occurring especially in a range 

 of freshwater and damp to marshy habitats. However, though often 

 abundant, they make relatively little showing as components of 

 vegetation — except sometimes in lakes or ponds, where they may 

 form a ' bloom ', or on stones in fairly rapid streams where they 

 provide a mucilaginous mat, in which other organisms such as 

 Diatoms thrive. They tend to be important in arctic regions where 

 they often form dark colonies on damp soil, in marshes, and about 

 the margins of tarns. Ecologically they may be of some importance 

 as the initial colonists (pioneers) on bare rock and other surfaces. 

 They have little economic significance except as nuisances in fouling 

 water supplies — to which they may impart a disagreeable odour and 

 taste, sometimes killing fish or even cattle. 



Thallophyta 



Chlorophyceae : These are a large and diverse group of Algae 

 (seaweeds, aquatic * slimes ', etc.) having chlorophyll located in well- 

 defined bodies called chloroplasts and not normally masked by other 

 pigments. Consequently the plants are usually green in colour and 

 the group is called the Green Algae. Characteristically they have 

 cellulose cell-walls and store food in the form of starch. What 

 appear to be the more primitive forms are microscopic, unicellular, 

 and either motile by flagella or non-motile, the cells occurring singly 



