IX. LEAVES AND THEIR FUNCTIONS 



In the horse-chestnut bud previously studied the brown scales 

 which cover the green scales of the bud can be shown to be like 

 in structure to a leaf (homologous to a leaf). This is seen better 

 in a near relative of the horse-chestnut, the bucke3'e, in which a 

 series of changes in form from 

 brown scales to green leaves 

 may be followed. 



Stipules. — The bud of the 

 tulip tree {Liriodendron tulip- 

 ifera) is an admirable source 

 of information as to the struc- 

 ture of a leaf.^ In the buds of 

 a tulip tree, however, the scales 

 seem to act as wrappers around 

 the little leaves and not to 

 correspond to the blade of the 

 leaf, but to an outgrowth of 

 the leafstalk or petiole below 

 it. The outgrowths at the 

 base of a leaf are known as 

 stipules. The scales in this case are stipules which have come to 

 protect the bud at a time when the delicate parts need protec- 

 tion most. These stipules are present as scales in many other 

 buds. Frequently, when the leaf becomes able to care for itself, 

 the stipules fall off and disappear. Such a bud is the elm, as 

 can be seen by a careful dissection. Stipules usually are paired. 

 Notice the notched appearance of the scales as you go inward. 

 Most leaves do not show the stipules well, although a complete 

 leaf is supposed to be supplied with them. They are well devel- 

 oped in the rose leaf. 



" See Hvinter and Valentine, Manual, page 37. 



123 



Palmately-veined leaf of the maple. 



