CHAP. XI. 
CIRCULATION OF THE SAP. 
829 
If a branch is cut half through at the same season, it will be 
found that, while the lower face of the wound bleeds copiously, 
scarcely any fluid exudes from the upper face ; from which, 
and other facts, it has been fully ascertained that the sap rises 
through the wood, and chiefly through the alburnum. Ob- 
servations of the same nature have also proved that it descends 
through the bark and liber. But the sap is also diffused 
laterally through the cellular tissue, and this with great ra- 
pidity; as will be apparent upon placing a branch in a 
coloured infusion, which will ascend and descend in the 
manner just stated, and will also disperse itself laterally in all 
directions round the principal channels of its upward and 
downward route. In trees this lateral transmission takes 
place chiefly through the medullary rays, which keep up a 
communication between the bark and the heart-wood, and 
convey to the latter the secretions which the former may have 
received from the leaves. 
With regard to the vessels through which this universal dif- 
fusion of the sap takes place, it has already been stated that its 
upward course is always through the woody tissue, and partially 
also through the vasiform tissue ; and that it passes downwards 
through the cellular system in part, and the woody tissue of 
the bark in part. But there can be no reasonable doubt that 
it is also dispersed through the whole system by means of 
some permeable quality of the membranes of the cellular 
tissue, which is invisible to our eyes, even aided by the most 
powerful glasses. It has also been suggested that the sap finds 
its way upwards, downwards, and laterally through the inter- 
cellular passages which exist at the points of union of every 
individual elementary organ. That such a channel of com- 
municating the sap is employed by Nature to a certain extent 
I do not doubt, especially in those plants in which the inter- 
cellular passages are very large ; but whether this is a universal 
law, or has only a partial operation, is quite unknown, and is 
not perhaps susceptible of absolute proof. 
The accumulation of sap in plants appears to be attended 
with very beneficial consequences, and to be deserving of the 
especial attention of gardeners. It is well known how weak 
and imperfect is the inflorescence of the turnip tribe, forced to 
