A MANUAL OF BOTANY 103 



The green algae. Observe for spirogyra, zygnema, and all 

 other green (as distinguished from tlie hlue green forms, as 

 oscilhitoria) alga3, that the water in which they grow, how- 

 ever unsightly the floating plants may appear, seems to be 

 very clear and bright, and is, in fact, reasonably pure and 

 well adapted to animal life. How do you explain this by 

 the results of the preceding studies? What relation may 

 these algre have to animal life? How may the pond scums 

 be injurious to man? 



Plants living in water are called hydrophytes. How does 

 their environment benefit them ? Carefully consider this 

 subject, for it brings to you a new field of work called 

 Ecology, or the relation of a living organism to all its sur- 

 roundings, mineral, plant, and animal. These algte are the 

 lowest members of the water association. 



Type 7. Desmids 



Habitat. These very small algae are more or less common 

 in sluggish streams, ponds, and ditches, where water plants 

 are plentiful. They may be obtained in some abundance by 

 dipping water from such situations, particularly from near 

 the bottom, and straining it through a fine meshed cloth. 

 After straining it through the cloth, put the sediment into 

 bottles with a little water. 



Structure. As these plants are practically invisible to the 

 unaided eye, the compound microscope is necessary. Examine 

 the material provided, (Ij^) <^r Q^P)) ^^^^ seek for small sym- 

 metrical objects, green in color. Study carefully this 

 symmetry of form. How does it manifest itself? Ex- 

 amine the cell wall for any mark or peculiar feature that 

 would cause the symmetry. How is the chlorophyll ar- 

 ranged ? Is it in any way covered up or concealed ? Try 

 to remove it by repeated treatment with alcohol. If you 

 are able to decolorize it, apply dilute iodine and observe 

 the results in the remaining cell contents. 



