120 LABORATORY AND FIELD EXERCISES 



Determine with care the thick and thin 

 areas of the wall. Which area constitutes 

 the greater portion of the entire wall of the 

 tracheid? What is the function of each in 

 the work of the tracheid and the stem? 

 (e) Summary. Draw and discuss briefly the struc- 

 ture and functions of the tracheids as com- 

 pared with the ducts of the higher plants. 

 10. Asexual reproduction. 



a. The sorus. Observe the lower (abaxial) surface of fern 

 leaves for clusters of fern sporangia, or sori. 



(1) What is the position of the sori with reference to 



the veins ? Can you think of a reason for this 

 position ? Are the sori numerous or infrequent 

 on the leaves that you are examining? What 

 advantages has the fern sporophyte over the moss 

 sporophyte in the production of spores ? See text 

 discussion for the number of fern spores pro- 

 duced by a single sporophyte annually. 



(2) Structure of the sorus. With hand lens and low 



power of the microscope determine the structure 

 of the sorus. Has it a covering ? Do sori of dif- 

 ferent ages have a different color and appearance ? 

 To what is this difference in color due ? Where 

 there is a membranous covering (indusium), note 

 its form in young sori, and the method by which 

 the brown sporangia became exposed to the air 

 in older sori. If different species of ferns are 

 available, observe sori and indusia of different 

 shapes and forms. Draw a single pinnule of a fern 

 to show the form, structure, and distribution of 

 the sori. Compare the indusia in your specimens 

 with those shown in the text (Figs. 168 and 176). 



