THE PLANT-BODY 15 



Colonies. Sometimes, among the lower plants, unicellular indi- 

 viduals are associated in colonies of very definite form, in which 

 the originally independent members may become intimately grown 

 together so as to simulate a tissue formed from the repeated fission 

 of an original cell (Fig. 3, D). 



The result of fission in a unicellular organism is the production 

 of two complete individuals. If, however, instead of separating 

 as soon as the division is completed, the cells remain together, and 

 fission is repeated in these cells in the same plane as before, the 

 result is a chain of united cells, which increase in length as the cells 

 undergo repeated division. This is really what happens in the next 

 type of plant-body the simple filament or cell-row, a type that is 

 very common among the lower water-plants, or Algae, such as Spi- 

 rogyra or Conferva. In these the plant-body is a row of perfectly 

 similar cells which arise from the repeated transverse division of 

 a single cell, and its descendants. Every cell being similar, it might 

 be almost as well to speak of such a filamentous Alga as a colony 

 of unicellular individuals. The life-history of such a form as Con- 

 ferva, for example, shows that the plant passes successively through 

 a free-swimming stage, followed by a unicellular stationary condi- 

 tion, which by repeated transverse fission develops into the cell- 

 row of the adult plant. Other forms, e.g. Spirogyra, never have a 

 free-swimming condition. 



It is not uncommon for some of the filamentous Green Algee to 

 remain for a considerable time in the unicellular condition, in which 

 they divide rapidly, the cells separating after division and closely 

 resembling the permanent condition of true unicellular Algae with 

 which they are easily confused. These stationary cells may either 

 grow directly into a filament, or they may- first assume again the 

 free-swimming condition previous to the formation of a filament. 



Indeed, the life-history of many of the filamentous Algae repeats 

 what was probably the process of evolution of these forms from the 

 free-swimming unicellular organisms from which we may fairly 

 suppose they originally came. 



Filamentous Plants 



While the simplest type of filament is that in which all the cells 

 are alike and there is no distinction of base and apex, there are 

 other forms, e.g. (Edogonium (Fig. 5), in which the filaments are 

 attached by a more or less modified rootlike cell, whose base corre- 

 sponds to the fonvard end of the zoospore from which it grew. 

 There is here a beginning of the specialization found in higher 

 plants. Of the two cells formed by the first division of the germi- 

 nating spore, the lower is at once set apart as a mere organ of attach- 



