CHAPTER III. 



THE THALLUS OF THE HIGHER ALG^. 



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

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

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

 gate. When Monostroma 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 algae. 

 The broader forms are sometimes 20-25 cm - wide. 



Ulva, a near relative, develops in much the same way, 

 but at least one series of divisions occurs in a third plane, 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 separate from 

 each other, as they do in Entero- 

 morpha, a hollow, sac-like body is 

 formed. 



So, from the linear aggregates, 



we pass through superficial to solid 



aggregates of a broadly extended 



form. 



The transition from linear to solid aggregates of slender 



26 



Fig. 34. — A small plant of Ulva 

 lactuca, the sea lettuce, show- 

 ing thallus, and rhizoid for 

 attaching it to rocks. Natural 

 size. — From Bessey. 



