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 
