FIXED PLANTS 127 



brandtia), or have fronds entirely solidified with car- 

 bonate of lime (Lithothamnium, Corallina). 



The diversity of their " thallus " (as their fronds are 

 usually called) is enormous ; some are so small that 

 they nearly escape our notice, and others so large that 

 stem and leaves have a length of hundreds of feet. 

 But however great their outward diversity may be, 

 their inner structure is comparatively simple, and built 

 up of either thin- or thick-walled cells. Nowhere do 

 we find a differentiation of these cells into vascular 

 bundles, as in flowering plants. Neither have algae 

 any true roots that can be compared to the roots of 

 flowering plants ; their holdfasts are in many cases a 

 few ' rootlets or a whorl of so-called hapteres ; other 

 algae fix themselves with a smaller or larger disc on 

 the substratum, and algae growing in sandy bottom 

 have long, hairlike, much-branched rootlets which 

 fasten themselves to the grains of sand, and enclose so 

 many of them that, when the plant is withdrawn from 

 the sand, its rootlets present the appearance of a large 

 lump. 



Algae have various colours, and in accordance with 

 these they are divided into : 



1. Blue algae — Myxophyceae or Cyanophyceae. 



2. Green algae — Chlorophyceae. 



3. Brown algae — Phaeophyceae. 



4. Red algae — Rhodophyceae or Florideae. 



We must bear in mind, however, that, though this 

 division is right in the main, there are many algae that 

 do not show the colour of the group to which they 

 belong. Blue algae living in deep water may become 

 red ; red algae may turn purple, brown-red, green, or 

 even yellow, on being more or less exposed to the glare 

 of the sun ; but with a little practice the group to 



