88 TYPE S'PUDrKS 



B. Scrape off some of the moistened Pleurococcus and mount in 

 water, well teased out. Examine under l.p. to find favorable 

 material. Under h.p. draw : 



1. Groups of cells in outline, showing their loose attachment 

 to one another, except just after cell division, when the 

 (laughter cells are to be found in pairs. 



2. A single large cell, showing the cell wall and the proto- 

 plast ; a nucleus can often be distinguished in the center 

 of the protoplast, and the chlorophyll is generally held in 

 a single large cliromatophore which may or may not have a 

 pyrenoid. These points are brought out more clearly by 

 staining with iodine. 



Questions. In what respects is the cell structure of Pleuro- 

 coccus higher than that of the blue-green algse ? What is the 

 life history of Pleurococcus ? Is it easily killed by winter's 

 cold and summer's drought, judging from its appearance on 

 trees and in other situations ? Make a study of the distri- 

 bution of Pleurococcus on a tree trunk, noting the limits of 

 growth and the regions of its greatest luxuriance. Try to 

 determine the reasons for the limits of growth. 



79. Sphaerella or Volvox (App. 10). These types and others of the Volvox 

 family, when available, are especially interesting for their life habits and 

 cell structure, and in the higher types for the complex cell colonies and 

 methods of sexual reproduction. 



A. In water swarming with Sphaerella note the reaction of the organism 

 to light when the vessel is placed near a window^ 



B. Under h.p. study the movements of the cell and the cell structure, 

 later killing and staining with iodine as described for Euglena 

 (Sec. 67, B). Note and draw : 



1. The thick, somewhat gelatinous cell wall. 



2. The protoplast with two cilia.^ red pigment spot, and large chromato- 

 phore. AVhich is the forward end as the organism swims ? 



C. Should gametes be developed and begin to conjugate, or should the 

 large vegetative cells form thick-walled resting cells, these processes 

 may be studied. 



D. Volvox as an example of a very highly organized cell colony may be 

 compared with Sphaerella or other one-celled forms of the same family. 



