76 SEAWEEDS 



the frond, these sori are more or less localised, though 

 usually of variable and irregular outline. The 

 thallus is in nearly all cases of considerable size, in 

 some of gigantic proportions exceeding in length 

 all other plants (Macrocystis) and attaining a girth 

 (Lessonia) comparable with that of trees. The brown 

 tangles of our seas are typical representatives of the 

 order. In all cases except Chorda there is a 

 differentiation of stalk and blade, and the growing- 

 point, which is an intercalary group of meristematic 

 cells, is situated at their junction. In point of 

 differentiation of the vegetative organs the order 

 rivals the Fucacew, and generally exceeds it in 

 stature. Cryptostomata with septate paraphyses 

 occur in Saccorhiza, Adenocystis, and Ulopteryx, while 

 tufts of such paraphyses not within cryptostomata 

 are found on the fronds of Alaria. 



The Thallus. The order has been divided by Mr. 

 Setchell into three tribes, according to the methods 

 by which the complexity of form of the adult plants 

 is attained. The differences in question naturally 

 arise at the intercalary growing-point, and the three 

 types of growth are the Laminaria type, the Lcssonia 

 type, and the Alaria type. 



The Laminaria type, to which the genera Chorda, 

 Saccorhiza, Agarum, Thalassiophyllum, Costaria, 

 Cymathcerc, and Arthrothamnus conform, is charac- 

 terised by unbranched fronds (except Thalassio- 

 phyllum) and by a simple unmodified plane growing- 

 point situated at the place where the stalk expands 

 into the blade, with nothing in its appearance to the 

 naked eye indicating the fact that here the stalk is 



