48 PLANT LIFE 



directly illuminated cells which take an 

 active share in photosynthesis. What is the 

 use, then, of those vast numbers of internal 

 cells which lie beneath the outer surface of 

 a large seaweed, and constitute its main 

 bulk ? Let us examine one of the big sea- 

 weeds, for example Laminaria, which form 

 the large leathery strap -like plants growing 

 below the tide limits. We shall find it 

 consists of a stout stalk, firmly adhering by 

 a specialised base to the rocks, and thinning 

 out abruptly above to form the flattened 

 frond. The cells which compose the plant 

 are by no means all alike, and at least three 

 different kinds can be distinguished. First 

 there are the crowded, rather small ones, 

 forming the superficial layers. These are 

 those chiefly concerned in photosynthesis. 

 Beneath the outer layers are other cells, 

 larger and more irregular in shape. These 

 are, partly at least, concerned in storing up 

 the surplus products of photosynthesis. 

 Thirdly, in the more central regions of the 

 massive stalk are to be found strands of 

 very much elongated cells which clearly serve 

 as conducting elements. In some of the larger 

 seaweeds the cross partition walls between 

 these cells are visibly perforated, thus admit- 

 ting of still easier passage of soluble contents 

 along their course. Some of these large 

 brown seaweeds recall the habit of our 

 terrestrial plants in that they even throw 

 off their " leafy " portions periodically, and 



