OF SAP IN PLANTS. 43 



interior of the wood of the main shoot, and here again at the 

 boundary between the outer and inner layers of wood (but espe- 

 cially in the latter), while in the bark and liber it had not reached 

 the ringed portion, much less passed beyond it. The fluid had, 

 moreover, not merely descended, but also ascended, in the main 

 shoot, and in the same region of the woody system. In the 

 absorbing twig itself it likewise passed into the leaves situated 

 below the absorbing leaves, and could readily be detected in the 

 unreliable spiral vessels of their petioles. It must be observed 

 that the two layers of the wood differed very much in their whole 

 character ; the outer was gorged with sap and evidently still in 

 active process of development (June 27th), while the inner was 

 white and dry, and therefore much better fitted to convey the 

 crude juices. It therefore only remains remarkable, that the fluid 

 which penetrated most easily in the medullary sheath of the twig, 

 left the neighbourhood of the pith in the main (older) shoot, and 

 passed to vessels situated further out. I observed the same in 

 Salix acuminata, Smith, in which the solution was traced 

 through three communicating generations or systems of branches. 

 The still green absorbing twig behaved as above ; the shoot from 

 which this arose possessed three layers of wood ; in this also the 

 fluid had descended chiefly near the pith. The second shoot 

 passed into (or arose from) a third thicker branch in the wood, 

 in which four layers could be distinguished, and here the fluid 

 had passed down at the boundary between the inmost and the 

 next succeeding layer of wood, and not next the pith. It perhaps 

 would not be erroneous to attribute this circumstance to the 

 difference of the annual course of growth, assuming that the 

 fluid always kept to one and the same tract, to vessels of the 

 same age, in passing from the youngest shoot into the older. 

 At all events this is not contradicted by the observed occurrence 

 of several layers of wood, for I have seen distinctly (in Salix 

 alba) that at least three succeeding systems (or generations) of 

 shoots may be developed in one and the same year, the lowest 

 and thickest containing two clearly distinguishable layers of 

 wood, which however were indicated even in the last and thinnest 

 (in the beginning of July). In order therefore to obtain a surer 

 basis for the decision of the differences of age in the young 

 systems of branches, I examined the condition of that small 



