THE ALG^E 193 



The growth in length of the thallus takes place with 

 the aid of a definite apical cell, by the division of which 

 all the tissues originate. Such a cell is situated at 

 the apex of each branch, and lies at the bottom of a 

 slit-like depression, which can be detected on examining 

 the tip of the branch with a lens. When branching is 

 to take place, the apical cell simply divides into two 

 by a wall down the middle ; so here we have a true 





FIG. 82. Conceptacle of Pelvetia, in median section, o, one of 

 the oogonia, each of which contains two ova. Surrounding 

 tissues of thallus also shown. Magnified about 10. (After 

 Thuret and Bornet. ) 



instance of dichotomy, the two branches being on exactly 

 equal terms from their first origin. 



Adventitious shoots arise chiefly as the result of 

 accidental injuries. The internal cells lying beneath the 

 wound are stimulated to renewed growth and division, 

 and give rise to a new thallus which may become an 

 independent plant. Thus the Alga ensures itself against 

 permanent loss in consequence of violence, for the 

 damaged parts are replaced by fresh and vigorous shoots. 

 The formation of these new growths, if they become 

 13 



