LECT. X.] ANATOMY OF LEAVfiS. 573 



side of the midrib, at the very base of the ex- 

 pansion, and bounding the margin of the leaf. 

 This receives supplies from the arches of the costse, 

 as it passes onwards, until it unites with its fel- 

 low at the apex of the leaf. Such is the general 

 distribution of the vascular fasciculi in the leaves 

 of dicotyledonous plants. It would be incon- 

 sistent with any elementary plan of instruction, to 

 enter largely upon the consideration of the various 

 circumstances which modify the distribution of 

 these fasciculi ; and, therefore, I shall examine the 

 modifications depending upon two states only of 

 dicotyledonous leaves : 1. When the leaf or its ex- 

 pansion is thin or membranaceous ; and, 2. when it 

 is thick QY fleshy. 



1. The general distribution of the vascular 

 fasciculi, in thin leaves of dicotyledonous plants, 

 in the majority of instances, resembles that which 

 we find in the skeleton of the leaf of the Indian Fig 

 tree. The vessels of the costse proceed from the 

 principal fasciculus of the midrib, and run between 

 the laminse of cuticle, embedded in the cellular mat- 

 ter, in cords which form visible elevations on the 

 back of the leaf, and corresponding furrows on its 

 face. Each fascicle consists of spiral and proper 

 vessels throughout all its ramifications; and, in 

 whatever manner these vessels are arranged in the 

 fasciculi, the spiral and proper vessels are always 

 associated and, in general, tangent. This arrange- 

 ment is common both to sessile and to petiolated, to 



