THALLOPHYTES 



271 



The Diatoms (Fig. 123) are unicellular forms that ai-e some- 

 times classed with the conjugales because of their method of 

 reproduction and sometimes with the PJuwophyceoB because of 

 their brown color. They are very small and very abundant in 

 fresh and salt water and on wet soil and also in both cold and 

 hot waters. They are exti-emely variable in shape and the cell 

 wall becomes impregnated with silica, thus forming a very hard 

 and durable shell. This shell consists of two parts, the one fit- 

 ting over the other very similar toi the lid on a pill box. They 

 have very delicate markings which are characteristic of the 

 species. The fossil shells are often found in very great abun- 

 dance, sometimes forming very large deposits of silicious earth. 

 This diatomaceous earth is an important article of commerce 

 and is used extensively in the 

 manufacture of dynamite 

 and scouring soaps. 



Some diatoms axe at- 

 tached, while others are free 

 swimming. The power of 

 locomotion is probably due to 

 very delicate strands of pro- 

 toplasm which p r o j e c t 

 through small openings in the shell. iS' on-sexual reproduction 

 is by cell division, one half of each new cell taking one-half 

 the old shell and forming a new half shell which fits into the 

 old one. Of course, the daughter plant cannot be larger than 

 the parent shell . and therefore we have the development of a 

 number of small individuals. However, after a certain amount 

 of reduction, a cell may lose its shells and become an auxo- 

 spore {i.e., an enlarged spore of naked protoplasm) which 

 has the power of growing and producing new shells of full size. 

 Sometimes the protoplasm of two individuals unites (conju- 

 gates) to form an auxospore. Sometimes four daughter cells are 

 formed from two cells and unite in pairs. 



The Stoneworts (Charales) are sometimes classified as the 

 tighest group of the algse, but some students of botany consider 



Fig. 123. — Common types of Diatoms. 



