LEAVES 51 



the middle of the bud.i The cell contents may, in great part, be removed, 

 and the sections rendered more transparent by treating them for some time 

 with strong potash solution, which is then to be washed out, and the sections 

 placed in concentrated acetic acid. They may be examined in acetic acid or 

 in a solution of potassium acetate. 



Examine the sections with a power of 200-300 diameters and make out 

 the way in which the growing apex of the stem is capped by a series of 

 layers of cells as follows : 



A. On the outside a single layer, the dermatogen, from which the epi- 

 dermis is developed. 



B. Beneath this the periblem, four layers of cells from which the bark, or 

 cortex, is formed. 



C. Within the layers of the periblem, the plerome, out of which the axial 

 fibro-vascular bundle of the stem is for the most part formed. 



Make a drawing to show the relations of all the parts above described, and 

 (lower down on the stem) the origins of the leaves. 



LEAVES 



39. The elm leaf." * * 



A. Sketch the leafy twig of elm that is supplied to you. 

 Eeport on the following points : 



1. How many rows of leaves ? 



2. How much overlapping of leaves when the twig is held 

 with the upper sides of the leaves toward you ? What 

 would be the advantages or disadvantages of much over- 

 lapping ? Are the spaces between the edges of the leaves 

 large or small compared with the leaves themselves ? 



B. Pull off a single leaf and make a sketch of its under sur- 

 face, about natural size. Label the broad part the blade, the 

 stalk by which it is attached to the twig leafstalk or petiole, 

 the appendages at its base stipules. Study the outline of the 

 leaf and answer these questions (see Pnneiples, Appendix) : 

 1. What is the shape of the leaf as a whole ? 



1 Unless the students have had considerable practice in making sections it will 

 be better to purchase slides of microtome sections of some growing point. 



2 If this subject is taken up during the winter, it will be necessary to use gera- 

 nium or other leaves from the florists for the study of leaf anatomy, and potted 

 geraniums, begonias, lilies, etc., for leaf arrangement. 



