CHAP. xiir. 
CIRCULATION OF THE SAP. 
401 
in that part, undergoes but little alteration from its condition 
when it enters the roots, and does not partake of the deleterious 
qualities of the descending sap of the bark. Observations 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 rapidity ; 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 articulated bothrenchyma ; and that it passes 
downwards through the parenchyma, and woody tissue of the 
bark, and through the vessels of the latex. 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, invisible to our eyes, even 
aided by the most powerful glasses. It has also been sug- 
gested that the sap finds its way upwards, downwards, and 
laterally, through the intercellular passages. That such a 
channel of communicating the sap is employed by nature 
to a certain extent I do not doubt, especially in those plants 
in which the intercellular passages are very large ; but 
whether this is a universal law, or has only a partial operation, 
is 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 
flower before their fleshy root is formed ; and how vigorous it 
is after that reservoir of accumulated sap is completed. 
Knight, in a valuable paper upon this subject, remarks that 
the fruit of Melons which sets upon the plant when very 
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