LECT. X.] ANATOMY OF LEAVES. 575 



veys 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 re- 

 turning vessels, enters the bark, but not the 

 wood. This fact is beautifully illustrated by 

 placing young leafy twigs in coloured fluids. The 

 colour is seen passing tip from the stem into the 

 leaf through the upper portion of each fascicle; 

 while that part which consists of the returning 

 vessels remains perfectly free from colour. 



Such is the general arrangement of the vascu- 

 lar framework in the thin leaves of dicotyledonous 

 plants ; to examine the intimate structure of the 

 fasciculi, we must have recourse to the same me- 

 thod that we adopted in our examination of them 

 in the leaves of the two natural divisions we have 

 already investigated. Taking the leaf of the 

 Lilac, Syringa vulgaris, as an example of the 

 simple petiolated leaf; and placing a thin transverse 

 slice of the petiole under the microscope, we find 

 that the vessels are arranged in the following man- 

 ner. Close to the upper or channelled surface of 

 the petiole, we find three small distinct fasciculi 

 of spiral vessels (c. c. c. fig. 10, Plate 8), one im- 

 mediately 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 

 horseshoe shape, in the transverse section ; and 



