AN INTRODUCTION TO PLANT BIOLOGY 



75 



such a section hold the leaf between two pieces of elder-pith, or slices 

 of potato, or roll the leaf to form a rather tight cylinder. Keep the 

 edge wet with water and with a very sharp razor shave off a number 

 of very thin slices. Float these off the razor into water and with 



FIG. 30. Cross section of privet leaf, ep, epidermis; pi, palisade cells in 

 upper part of middle tissue; A;, deposited crystals; a, air-spaces; , vascu- 

 lar mid-rib; st, a stoma. (From Strasburger.) 



fine forceps or a small brush transfer these sections to a drop of 

 water on a glass-slide and place the cover-glass in position.) Com- 

 pare a transverse section with Fig. 30 and locate all the structures 

 in the cut-end of the leaf that was seen with the hand-lens. Note 

 that the epidermis is transparent and without green color, except 

 in the cells (the guard-cells) at the leaf- 

 pores. Compare with the epidermis which you 

 previously stripped off and examined in sur- 

 face view. 



Most of the cells in the middle-tissue have 

 green bodies (the chlorophyll-bodies, or chloro- 

 plasts). The compact upper part of this 

 middle-tissue is seen to be composed of elon- 

 gated cells (palisade cells) set closely together. 

 In the lower part of the leaf the cells are irreg- 

 ular in shape and there are numerous air-spaces. 

 Some of these spaces communicate with the 

 outside through the leaf-pores, and thus air 

 may enter the leaf and become widely distrib- 

 uted throughout the air-spaces. 



Bundles of small transparent cells in the middle-tissue are the 

 cut ends of veins, and the empty cells in them are tubes for carrying 



FIG. 30 a. Magnified 

 view of cells around 

 a stoma (s). a, air- 

 space; c, cells with 

 chlorophyll; lower 

 cells (e) epidermis. 

 (From MacDougal.) 



