CHAPTER XII. 



Minute Structure of Leaves ; Functions of Leaves. 



133. Leaf of Lily. — A good kind of leaf with which to 

 begin the study of the microscopical structure of leaves in 

 general is that of the lily.-' 



134. Cross-Section of Lily Leaf. — The student should first examine 

 with the microscope a cross-section of the leaf, that is, a very thin slice, 

 taken at right angles to the upper and under surfaces and to the veins. 

 This will evidently show : 



(a) The upper epidermis of the leaf. , 



(6) The intermediate tissues. 



(c) The lower epidermis. 



Use a power of from 100 to 200 diameters. In order to make out the 

 relations of the parts, and to get their names, consult Kg. 96. Your 

 section is "by no means exactly like the figure. Label properly all the 

 parts shown in your sketch. 



Are any differences noticeable between the upper and the lower 

 epidermis ? Between the layers of cells immediately adjacent to each ? 



The teacher can (after considerable practice) prepare such sections by 

 doubling the leaf crosswise once or twice, and then Slicing the required 

 sections from the end of the folded leaf, held firmly in the hand, if 

 necessary between bits of elder-pith to hold it in position. The razor 

 must be sharp, and the stroke made rather quickly and long.'-^ The 

 upper edge of a section may be distinguished from the under one by the 

 presence in the former of %ialisade cells, so called from their resemblance 

 to the high fence known as a palisade, made of stout stakes driven into 

 the ground. Fig. 96. Mount in glycerine for temporary use in class. 



135. Under Surface of Lily Leaf. — Examine with a power of 200 or 

 more diameters the outer surface of a piece of epidermis from the lower 

 side of the leaf.^ Sketch carefully, comparing your sketch with Pigs. 97 

 and 98, and labeling it to agree with those figures. 



1 Any kind of lily will answer. 



2 Consult Clark's I'mrtical Methods in Mirrot^copy. pp. 67-70. 



8 Xlie epidermis may be started with a sliarp knife, then peeled off with small 

 forceps, and mounted in water for microscopical examination. 



