ALG^ 



69 



including all the giant kelps, show only the simplest 

 possible form of reproduction, i.e. purely non-sexual 

 zoospores. Many of these larger kelps show an external 

 differentiation which simulates closely the parts of the 

 higher terrestrial plants. A definite axis, with lateral 

 leaf-like outgrowths, has 

 its base modified into a 

 mass of firm root-like or- 

 gans, forming a most 

 efficient holdfast or an- 

 chor, which, in some of 

 the largest kelps, when 

 torn away may carry 

 with it a mass of rocks 

 and shells weighing sev- 

 eral hundred pounds. 



The leaves of the large 

 kelps are often several 

 metres in length, and 

 although structurally 

 they differ widely from 

 those of the higher 

 plants, yet functionally they must be considered as 

 equivalent to these. It is in these organs that the 

 greater part of the chlorophyll-bearing cells are situ- 

 ated. The peculiar floats or air-bladders found in these 

 plants are formed by the accumulation of gases within 

 certain parts of the plant, resulting in a distention of 

 the thallus at these points, but the details of their 

 development cannot be given here. 



While some of the forms, including the larger kelps, 

 appear to possess only non-sexual zoospores, others, like 



Fig. 18 (Fucacese) . — A, a fragment ol the 

 common gull-weed, Sargassum, show- 

 ing the definite stem and leaves, and 

 the berry-like floats, v ; B, the egg of 

 the common rock-weed (Fucus vesi- 

 culosus), being fertilized by the minute 

 biciliate spermatozoids ; C, a single 

 spermatozoid more highly magnified. 



