DEVELOPMENT OF PLANTS 183 



asexual plant, asexual generation or sporophyte, since this plant 

 can only produce zoospores. Among the green algae this dis- 

 tinction is not very manifest, the Ulothrix plant for example being 

 the sexual plant or gametophyte and the gametospore or the 

 little plant derived from it is the asexual plant or sporophyte, 

 in this case being merely a single cell. It is very manifest, how- 

 ever, that this single cell or plant is decidedly different from the 

 Ulothrix plant because it cannot develop into the Ulothrix plant, 

 but can only produce zoospores. So we see that there are two 

 generations in the life history and that they differ radically in 

 their natures and possibilities of growth. In higher types of 

 plant life it will be seen that the gametospore tends to develop 

 into a more and more complex body or plant and it will be one 

 of the most interesting features of the work to watch the evolution 

 of this asexual plant. 



The higher members of the Chaetophorales illustrate the same 

 gradual differentiation of the gametes, as has been noted in the 

 motile forms of the Volvocales. The female gametes become 

 distinguishable because of their larger size and shorter period of 

 motility, while the male gametes are small, owing to the large 

 numbers that are found in a cell or because of the small size of 

 the cells in which they are developed. Finally, in several of the 

 genera we find but a single female gamete which remains motion- 

 less in the cell. This condition is very well illustrated in one of 

 the genera of the next order. 



70. Order d. Siphonales or Tubular Green Algae. — This order 

 includes a large number of odd forms that are filamentous in 

 character and they differ from all other algae in that the filaments 

 contain numerous nuclei, but with rare exceptions no cell parti- 

 tions. Such plants are called coenocytes. They assume various 

 forms and often resemble a small plant with stem, root and leaf, 

 but in all these cases the plant is essentially a huge cell or tube 

 without partitions and containing numerous nuclei. Most of the 

 Siphonales are marine, but one genus, Vaucheria, is well repre- 

 sented in shallow streams, damp places and on the earth of flower 

 pots in greenhouses, where it forms rather coarse green felt-like 

 masses. The plant body consists of long tubular threads, often 



