THALLOPHYTA— ALG^ 35 



unicellular and multiply by ordinary fission ; each cell- division 

 so produces a new plant. In others the cells do not separate from 

 each other, but the division proceeds until a long filament of cells 

 is formed, each cell being generally physiologically independent, 

 though connected anatomically with the others. In other cases 

 the cells divide in two planes, forming a plate of cells. Some- 

 times the imicellular forms do not separate from each other 

 after division, but remaui connected by a common mucilaginous 

 eeU-waU, forming a colony or ccenobium. In other Algae the 

 structure is a coenocyte, no cell- walls being formed in the in- 

 terior of the filament, which thus appears unicellular. In the 

 larger ccenocytes, as CmUerpa, the cavity is often crossed by 



Fig. 785. 



Fig. 785. Vertical section of a female couceptacle of Fucys vesiculostis con- 

 taiuiug oogonia and parapliyses. Alter Tliuret. 



trabeoulse of modified cellulose. In the higher forms the plant is 

 multicellular, and shows a rudimentary differentiation of tissues ; 

 a kind of epidermis often being distinguishable from a central 

 tissue, which is sometimes furnished with sieve-tubes. In the 

 stalks of some Laminarias there is a secondary meristem which 

 increases the thickness of the part. In many of the PhEeophycese 

 the internal cells are hypha-like, and loosely arranged with 

 mucilaginous walls and large intercellular spaces {ficf. 785). 

 There is sometimes a fair amount of morphological differentia- 

 tion ; generally the plant body is a thaUus, but frequently 

 rudimentary roots can be distinguished, serving as organs of 

 attachment. The plant is often attached to its substratum by 



n 2 



