LECT. X.] ANATOMY OF LEAVES. 555 



those of the bark, the disposition could not well 

 be otherwise, seeing that the relative position of 

 the upper and under disk of every leaf, to the 

 centre of the stem, is exactly that of the albur- 

 num and the bark. In leaves, however, which 

 stand vertically, or have no distinction of sur- 

 faces, the situation of the spiral vessels is either 

 the reverse, or in the centre of the entire vessels : 

 anatomy thus confirming the idea of the close 

 affinity of such leaves to stems. 



I have already stated that the bundles and 

 threads of vessels, in leaves belonging to this di- 

 vision of the class of leaves under consideration, 

 run in longitudinal lines. These are not exactly 

 parallel, but approach both at the base and the 

 apex of the leaf; and, also, communicate la- 

 terally in their course by small threads, given off 

 at irregular intervals. This structure is easily de- 

 monstrated by placing a small slice of the Lily 

 leaf (7, page 553), cut immediately withhi the cu- 

 ticle of the upper disk, under the microscope. 

 We perceive that the longitudinal bundles (a. a.) 

 are united by transverse threads (b. b.) ; and this 

 is the case at irregular intervals throughout the 

 whole extent of the leaf. 



These transverse fasciculi, however, are not 

 threads simply detached from one longitudinal 

 layer, and coming into contact with another, as 

 was supposed by Dr. Grew to be the case in all 



