THE PTERIDOPHYTES. 195 



Examine a piece of the epidermis torn from the upper 

 side of the leaf. Draw a few cells. State how it differs 

 from the epidermis of the lower side. 



7. Hold pieces of the leaf between pieces of pith and 

 cut cross sections. Draw a portion of a section showing 

 a fibro vascular bundle. Beginning on the upper side, 

 notice the epidermis, a single row of cells ; under these, 

 green cells standing perpendicularly to the epidermis, 

 columnar parenchyma; then other similar green cells 

 irregularly arranged and having intercellular spaces be- 

 tween them ; on the lower side of the leaf a single row 

 of cells forming the epidermis. Find sections through 



FIG. 121. A fertile leaflet of Pteris aquilina seen in cross section, sp, spo- 

 rangia ; in, indusium ; ms, mesophyll or loose parenchyma ; mr, midrib ; 

 v, veins. (After Sedgwick and Wilson.) 



stomata in the lower epidermis, and notice that each 

 stoma opens into a large intercellular space, or breath- 

 ing chamber. The chlorophyl-bearing cells of the interior 

 of the leaf are called mesophyll. Do they contain starch? 

 8. Examine the sporangia. These may be obtained by 

 dissecting off the indusium and scraping the sporangia 

 from the margin of the leaf. A sporangium closely re- 

 sembles an interrogation point (?) in shape. Notice the 

 pedicel, or stalk, the annulus of thick-walled cells which 

 extends from the top of the pedicel up one side and over 

 the top of the sporangium, and the thin-walled cells in- 

 closing the body of the sporangium. Draw. When the 

 sporangium is ripe, the thick-walled cells of the annulus 

 become dry and contract; this causes the thin-walled cells 



