232 BOAED OF AGRICULTURE. 



Having thus demdnstrated that crude sap ascends chiefly 

 in the sap-wood of exogenous trees, let us now consider a 

 few facts which appear to prove that there is a counter-move- 

 ment of elaborated sap which is for the most part confined to 

 the bark. 



It is well known that if a narrow ring of bark be removed 

 from the trunk of a tree between the leaves and the roots, 

 then the deposition of wood ceases below the girdled place, 

 though above it the growth for the season ensuing will be 

 quite normal. This proves beyond dispute that the wood 

 cannot convey that portion of the elaborated sap which is 

 essential to growth, and that it can be conducted only by the 

 tissues of the bark, or the imperfectly-developed tissues of 

 the cambium between it and the perfectly-formed wood. 

 Nevertheless, there is free communication in a transverse 

 direction for the crude sap and for some of the elaborated 

 substances between the wood and the bark, probably by 

 means of the medullary rays which connect the two. Thus 

 only can we account for the fact that the bark below a girdled 

 place remains alive long after the deposition of wood ceases, 

 and also for the circumstance that starch and sugar, which 

 must originally come from the leaves, are found either accu- 

 mulated in the cells of certain stems and roots, or existing in 

 the sap which flows or is expressed from their tissues. If we 

 shave off, little by little, the bark of a maple when the sap is 

 flowing freely, we shall observe no exudation from any por- 

 tion of the liber, even, but as soon as the whole of this is 

 removed, the sap issues from every part of the surface. 



Again ; those who work with mill-logs tell us that in the 

 spring the bark becomes soft and loose, precisely as if the 

 tree were standing, at least in the case of some species. 

 Sometimes logs and poles, cut for fences, will sprout and 

 actually produce shoots with foliage, the sap of which must 

 be derived wholly from the timber, and must, therefore, pass 

 from the wood to the bark. 



Mr. Wm. F. Flint has sent us a piece of a red maple slab, 

 which he found on moist ground, under a pile of wood, and 

 which had thrown out at the ends and sides a callous a 

 quarter of an inch thick, precisely like an ordinary cutting of 

 a grape vine. Here we have an instance of growth without 



