THE STEM AND THE LEAF 



69 



Stil 



tti 



which active cambium remains, are said to be open. The 



palms and similar monocotyledonous trees cannot form an- 

 nual rings of wood. There are, however, 

 a few tree-like monocotyledons in which 

 the trunk continues for years to increase 

 in thickness, and may reach great dimen- 

 sions, but these trunks do not thicken 

 in the same way as do the trunks of our 

 familiar trees. 



Many of the woody monocotyledons are 

 remarkable for the extraordinary length 

 and slenderness of their stems. The rat- 

 tans, for example, often climb for hun- 

 dreds of feet among the tops of tropical 

 forest trees. 



68. The parts of the leaf. It already 

 has been stated in section 11 that a leaf 

 consists of pet- 

 iole and blade. 

 A few words 

 may now be said 

 about the exter- 

 nal forms of ordinary leaves and the 



parts of which they consist. 



At the base of the petiole many 



leaves bear a pair of appendages 



called stipules (figs. 48 and 49). In 



some leaves, as those of the pansy, 



these form an important part of the 



total leaf surface. Not infrequently, 



a** in the black locust, the stipules of ayoling leaf> only ^^ ex _ 



have the form of thoms, one at each panded from the naked bud ; 



side of the base of the petiole. 



In general the form of the leaf 

 depends very much on the distribution of the groups of fibro- 

 vascular bundles known as veins. Most monocotyledons have 



FIG. 48. A young leaf 

 of wild black cherry 



bl, blade, or expanded 

 part ; sta, leafstalk ; sti, 

 stipules, or appendages at 

 the base of the leafstalk 



FIG. 49. Tip of a geranium 



(Pelargonium) shoot 

 /-/. blade of a leaf; bl', blade 



/, leafstalk; at stipules. Con- 

 siderably reduced 



