CHAPTER III. 



THE THALLUS OF THE HIGHER ALG^t. 



31. From linear to solid aggregates. — From the fila- 

 mentous algfe, whose body is a linear aggregate of cells, it is 

 but a step to those forms whose body is a superficial aggre- 

 gate. When Mutioslroma grows from the single cell as which 

 it begins life, the cell-divisions, instead of occurring succes- 

 sively in parallel planes, are made in two planes at right 

 angles to each other. The result is a single sheet of cells 

 forming a leaf-like thallus attached to stones or other algse. 

 The broader forms arc sometimes 20-25 '^™- "^^'idc- 



U/va, a near relative, develops in much the same way, 

 but at least one series of divisions occurs in a third i)lane, at 

 right angles to the other two, so 

 that the body of the sea-lettuce con- 

 sists of two layers of cells. As 

 fig. 34 shows, it is very clearly dif- 

 ferentiated into rhizoid and thallus. 

 If two such layers sei)arate from 

 each other, as thev do in Entero- 

 mur/ihd, a hollow, sai-like body is 

 formed. 



So, from the linear aggregates, 



Fif;. 34. — A small piant of fV7'(i , /^ ^ ^ 



An/nra, the sea lettuce, show- We pass through SUpcrpChll tO Solld 

 ing thallus, and rhizoid for 



attaching it to rocks. Natural aggregates of a broadl)' extended 



size. — I'Vuin Bessey. 



form. 

 The transition from linear to solid aggregates of slender 



26 



