82 PLANT-LIFE 



waters of temperate and cold coasts. It is in this 

 family that we have the most advanced type of sexual 

 reproduction among the Brown Algse; indeed, asexual 

 reproduction is entirely wanting — that is, if we leave 

 out of consideration the fact that detached fragments 

 of a thallus may develop into perfect plants. Pelvetia 

 canaliculata belongs to the Fucacese. We have already 

 noted a few of its external characters (p. 73), but as 

 we propose to take it as a type of the Wracks, it is 

 necessary that we should consider further points of 

 structure and the nature of its reproductive activity. 



In the thallus of Pelvetia we have a distinct advance 

 upon the thread-like row of cells joined end-to-end. A 

 cross-section examined under the microscope shows 

 numerous cells, which are arranged in accordance with 

 their peculiar functions. There is an outer ring of cells 

 containing chloroplasts masked with brown pigment; 

 this evidently forms a carbon-assimilating tissue. In- 

 side this ring there is a circle of large cells which are 

 lightly coloured; they probably assimilate carbon, but 

 not nearly so extensively as the cells of the outer tissue. 

 In the centre of the section there are elongated cells 

 which probably serve to conduct food material. At the 

 lower part of the thallus and in the attachment disc 

 the central cells are greatly elongated. They form a 

 tangled network and have strong walls; they serve to 

 strengthen the plant at a point where the action of 

 waves might tear it. The thallus grows in length 

 through the repeated division and subdivisions of a par- 

 ticular cell at the apex. If the thallus is injured at any 

 point, the cells near the wound are not merely active 

 in repairing the damage, but are stimulated to such a 



