52 THE PLANT WORLD. 



stained too deeply, wash in distilled water containing a few drops of 

 hydrochloric acid. When the stain has been washed out sufficiently 

 the filaments should be placed in 2 per cent formalin. One or two 

 filaments may be mounted on a glass slide in a drop of formalin (2 per 

 cent) and covered with a glass slip, the edge of which is coated with 

 Canada balsam to exclude the air. Such slides, if carefully handled, 

 will furnish permanent mounts. By this process the nucleus and pro- 

 toplasm stain pink and the thread-like connections between the nucleus 

 and pyrenoids are distinctly seen. 



The plant (filament) grows in length by the division of any or all 

 of the cells. As it breaks easily and each piece forms a new indi- 

 vidual, the plant multiplies rapidily. 



Reproduction takes place by conjugation. Two filaments lying 

 side by side send out little tubes one from each cell. The tubes from 

 two opposite cells meet between the filaments and unite. The walls sepa- 

 ratinof the interior of the two tubes dissolve and the contents of the 

 supplying cell is poured through the connecting tube into the opposite 

 or receiving cell where the zygote is formed. At the approach of 

 winter the plant dies, but the zygote, or resting spore, falls to the 

 bottom of the pond and lies there protected from the cold by the thick 

 wall surrounding it. In the spring it germinates and forms a new 

 plant. 



Micrasterias (Greek mikros, small, aster, a star). (Fig. 9.) 

 These starlike, unicellular plants are usually found among other algae, 

 as in the debris surrounding water plants. The disc-like cell is divided 

 bilaterally, usually l)y means of a more or less deep constriction around 

 the center of the cell. The wall is thick and gelatinous and is often 

 prolonged on the outside into horn-like projections in the form of rays, 

 knobs or warts. The plant is able to move slowly through the water, 

 it is thought, by means of small streams of protoplasm issuing from 

 the inside of the cell. 



Reproduction, as in Spirogyra, takes place by means of conjugat- 

 ing tubes between two individual cells, except that in this case, the 

 zygote or resting spore is formed and matured in the expanded tube 

 which connects the two individuals. 



The plant also multiplies by cell division. The isthmus or con- 

 necting ta))e between the two halves of a cell elongates and increases 

 in size. This tu])e is formed into two half cells and when almost the 

 size of the parent cell the two individuals part by means of the com- 

 pleted constriction in the middle of the tube. So one parent half- cell 

 and one daughter half-cell forms the new individual. 



University of Minnesota. 



