158 



BOTAWT. 



In this way often much light will be thrown upon the morphology 



of leaf parts* 

 (6) Among Dicotyledons it is generally best to select those whose 

 young leaves are least downy or hairy, 

 otherwise the difficulties of the examina- 

 tion are greatly increased. The lilac is 

 one of the best' for this purpose. Longi- 

 tudinal sections, prepared as in the ex- 

 amination of young stems, should be 

 made. 



(c) The young leaves in the winter buds 

 of the hickory are instructive, as showing 

 how compound leaves are formed. 



(d) The study of the arrangement of 

 leaves is most interesting in the twigs 



and cones of the Conifers, and the stems arid heads of the Composites. 

 The student should, however, before spending much time in the 



Fig. 135. — A part of a trans- 

 verse section of the 1' af of 8tipa 

 spartea in the position it as- 

 sumes— 2.€., witli what is really 

 the tipper surface turned toward 

 the earth. /,/, rib--, each con- 

 taining a fibro-vascular bundle ; 

 between these are the mas.^es of 

 chlorophyll-bearing parenchyma 

 (figured dark in the cut), ^" 



X 18. 



Fi? 136.— Transverse section of one of the ribs of the leaf of Sllpa spa/rtea. tp, 

 chlorophyll-bearing parenchyma ; «, s, portions of the epidermis containing stomata ; 

 Ac, Ac, hygroscopic cells, which contract when the leaf rolls up. The blanlc space on 

 the left shows the extent of the cavity occupied by chlorophyll-bearmg parenchyma. 

 X 125 — Ffom a drawing by the author. 



examination of the more difficult forms, study the twenty-sixth section 



of Sachs' " Text-Book of Botany," and the whole subject of the 



* In illustration of this, the Iris itself may be cited. Its leaf is 

 usually spoken of as made by the folding of its upper surface upon 



