THALLOPHYTA: ALGAE 95 



unicellular, have a very primitive cell structure, and reproduce by fission. 

 Because they live in a variety of habitats, including hot springs, they 

 may have lived on the earth before conditions were favorable for the 

 existence of other green plants. 



The algae are not a homogeneous assemblage l)ut embrace a number 

 of groups representing divergent lines of descent, all of which probably 

 have had a common origin. Advanced students of the algae try to trace 

 these lines of evolution, but we shall be concerned mainly with certain 

 general tendencies and with the progress that the group as a whole has 

 made. 



Development of Multicellular Bodies. The unicellular plant body 

 obviously represents the simplest condition of structural organization 

 and, necessarily, also the oldest. It is characteristic of all the blue-green 

 algae, flagellates, dinoflagellates, diatoms, and many of the green algae. 

 Unicellular plants may be either solitary or colonial, the latter condition 

 having arisen from the tendency of cells, following division, to remain 

 together for a while before separating. In the evolution of the algae, 

 close association of cells in a colony may have led to a dependence of the 

 cells on one another, with the resultant establishment of a multicellular 

 body. It is significant that, among the algae, no sharp distinction exists 

 between highly organized colonies and simple multicellular plants. This 

 intergradation strongly indicates that multicellular plants have been 

 derived from unicellular ones through the formation of colonies. 



Although the multicellular bodies of algae are very diverse in form, they 

 may be referred to three main types: filamentous, plate-like, and massive. 

 The filamentous type is most common, probably because it seems best 

 adapted to aquatic life. In such a body all the cells are in direct contact 

 with water and the absorbing surface is very large. Thus the absorption 

 of gases is greatly facilitated. The massive type of body, as exemplified 

 by many brown algae, is adapted to withstand the buffeting action of 

 waves and water currents along rocky seacoasts. In the simple multi- 

 cellular algae growth is intercalary, every cell having the power of divi- 

 sion. In branching forms growth is often limited to definite regions, such 

 as the terminal cell of each branch. In many brown and most red algae 

 growth occurs by means of an apical cell that cuts off a series of posterior 

 segments. 



An important evolutionary tendency exhibited by the algae is for 

 certain cells to become structurally differentiated in response to special 

 functions. It occurs in both colonial and multicellular forms. A simple 

 expression of this tendency is seen in those filamentous algae having the 

 basal cell modified as a holdfast. In many branching filaments the cells 

 of the branches are smaller than those of the main filament. The forma- 

 tion of sporangia and gametangia represents a specialization of certain 



