DEVELOPMENT OF PLANTS 179 



the original halves and one half of the isthmus. This latter 

 part gradually enlarges and forms the second half of the desmid. 

 Owing to the gelatinous character of the cell walls, the desmids 

 often become aggregated in masses and form chains of cells 

 (Fig. 108, £). 



69. Order c. Chaetophorales or Filamentous Green Algae. — 

 Here belong a great array of green algae that grow attached to 

 various objects and appear as simple or more usually as branched 

 filaments which frequently end in hair-like tips. They are of 

 common occurrence at the bottom or margins of springs, wells, 

 streams and ponds. Ulothrix may be taken as an example of 

 one of the simple forms of the Chaetophorales. Fig. no shows 

 that it consists of a simple chain of cells that divide and so bring 

 about the growth of the plant as in Spirogyra. However, it is 

 important to note that we have here a higher type of vegetative 

 structure than in any of the algae heretofore studied. This 

 advance appears in the one or more rather colorless cells at the 

 base of the plant which serve to anchor it to the substratum. In 

 other words, there is apparent a distribution of labor or the per- 

 formance of definite duties by special parts or cells of the plant 

 body. The evolution of higher forms comes about in much this 

 way. So long as each cell of the plant performs the same func- 

 tion, there can be little variation in the form and structure. But 

 as these cells are differently stimulated by their environment, 

 they tend to vary in character and so become adapted to the per- 

 formance of different functions. Perhaps in Ulothrix the con- 

 tact of the substratum acted as a stimulus, as Pierce found to 

 be the case in many algae. Thus the plant comes to consist of 

 different kinds of cells, each of which performs a different kind 

 of work. The complexity of the plant is due to the variations of 

 its cells which are thereby adapted to the performance of special 

 duties. In Ulothrix, we have one of the simplest modifications 

 of this nature, one or more of the basal cells becoming slightly 

 changed in form and contents and so adapted in anchoring the 

 plant. The other cells of the plant are practically alike, each con- 

 taining a single nucleus and chloroplast and therefore capable of 

 manufacturing food and forming the reproductive bodies which 

 are of the same simple type as noted in Sphaerella. 



