1 84 



PHAEOPHYCEAE 



growing region on each branch is ventrally situated in the terminal 

 flag or blade, and it is here too that splitting takes place to form the 

 individual laminae. The splitting is brought about by local gelatin- 

 ization of the inner and middle cortex together with a cessation of 

 growth in the epidermal area; this forces the adjacent tissues into 

 the gelatinized areas until finally the epidermis is ruptured. Two 



Fig. 124. Macrocystis pyrifera. A, young plant ( x ^). B, slightly older plant 

 with primary slit and tw^o secondaries ( x i). C, still older plant. D, young 

 plant. E, origin of blades at the apex. F, young plant. G, mature plant. 

 H, sporangial sori ( x yV)- I. transverse section of thallus showing ridges ( x 3-5). 

 J, surface view of holdfast of old plant showing flattened rhizome. (A, B, after 

 Brandt; C, H, J, after Setchell and Gardner; D-G, after Oltmanns; I, after 

 Smith and Whitting.) 



kinds of zoospore are recorded, large ones which give rise to the 

 female gametophytes and smaller ones which give rise to the male. 

 The appearance of true heterospory in such an advanced alga is a 

 feature of considerable importance because the phenomenon is 

 normally associated with the land plants. The eggs are reported to 

 be fertilized whilst still in the oogonium and if this is so then we 

 have here the only example among the brown seaweeds of the 



