POLYTRICHUM COMMUNE 79 



ness? Describe. Are stomata present? Why? 

 How is the midrib distinguished? Describe the 

 leaf-margin and apex. Describe any differences 

 in the two sides of the leaf. 



9. Describe fully the contents of a single cell, as 

 observed under high power. 



10. Illustrate by suitable drawings all features observed 

 under D, 7-9. 



The Stem. 



11. Study, under the low power, cross-sections of the 

 stem mounted in clearing fluid (or use prepared 

 slides) . 



12. Describe the tissues observed, and their relation to 

 each other. Compare the structure of the game- 

 tophyte-stem, as seen in cross-section, with that of 

 the sporophyte-stem, and name the tissues of the 

 former, using the terms given above (Z>, 6). 



13. Illustrate by a drawing, at least 50 mm. in diameter, 

 the structure of the stem as seen in cross-section. 



E. Non-sexual Reproduction: 



1. In some mosses a second gametophyte often devel- 

 ops from the tip of an older plant. This is called 

 proliferation. Frequently this may be repeated a 

 number of times, forming a chain of plants, each 

 younger one growing out of the apex of the next 

 older one. Examine the material at hand, and, if 

 such a condition is found, describe it, with drawing. 

 What kind of reproduction is this? 



2. Explain the advantage to the species of the elon- 

 gation of the seta. 



3. If stages in the germination of the spores are avail- 

 able, study this process. The structure imme- 

 diately developed from the spore is the protonema. 



