294 ANATOMY OF LEAVES. 



by the number of the leaflets ; in the Elder, we find 

 generally five ; and in the Horse Chesnut, from 5 to 7 

 or 8. It is, however, the inner part only of these fasci- 

 culi, or that which conveys the sap to the leaf, that is 

 given off from the wood, or rather from the medullary 

 sheath ; for the outer part, which consists of the proper 

 or returning vessels, enters the bark, but not the 

 wood. This fact is finely illustrated by placing young 

 leafy twigs in colored fluids. The color is seen pass- 

 ing up from the stem into the leaf through the upper 

 portion of each fascicle ; while that part which con- 

 sists of the returning vessels remains perfectly free 

 from color. 



Seen under the microscope, the following arrange- 

 ment of vessels takes place in a thin transverse slice 

 of the petiole of the Lilac, an example of a simple 

 petiolated leaf. Close to the upper or channelled sur- 

 lace of the petiole, we find three small distinct fasci- 

 culi of spiral vessels, one immediately within the cutis, 

 in the hollow of the channel, and one at each side ; 

 but the principal vessels constitute one large compound 

 fasciculus, in the centre of the petiole, which appears 

 of a horse-shoe shape, in the transverse section ; and 

 consists of one fasciculus of spiral vessels, and two 

 fasciculi of proper vessels. The spiral vessels, which 

 form the central fasciculus, are arranged in rays, 

 which are sometimes tangent, at other times separate ; 

 whereas the proper vessels constituting the two fasci- 

 culi, one of which is situated within, and the other 

 without the fasciculus of spirals, are irregularly im- 

 bedded in a pulpy parenchyma, and are readily dis- 

 tinguished by their greater transparency. The bark, 

 or true cutis of the petiole, seems, also, to consist 

 chiefly of several series of the same kind of proper or 

 returning vessels. In the various modifications of this 

 structure of the vascular system, in the petioles df 



