596 IMPROVEMENT OF PLANTATIONS, 
mals, for there is no heart, no organised internal viscus, which can 
propel the vegetable fluid through one set of vessels and receive it 
back again, by the channels of another set. There exists, particu- 
larly in the early spring, an ascending movement in the juices de- 
rived from the soil, and this can, in many instances, be distinctly 
traced. After the developement of the leaves, however, the flow of 
the sap appears to cease : that is, plants do not, after the leaves are 
expanded, evince any great tendency to bleed, when they are cut 
across. The movement of the sap, notwithstanding, is not in reaHty 
interrupted ; for it increases as the temperature of the season advan- 
ces, but as the leaves are expiratory organs, a large portion of the 
more aqueous part of the sap is either carried off, or undergoes a re- 
markable change by the operations of light and air upon it within 
their cellular membranous tissue. The ascending sap, as far at least 
as the taste is capable of determining, can scarcely be distinguished 
from pure water. It holds however in solution, matter, which renders 
it liable to undergo speedy fermentation, and to become either acid 
or putrid. The motion of the sap, strictly speaking, is that of ascent, 
but it is not through vessels tubular throughout their whole structure 
that the sap ascends : it is through the cellular substance (parenchyma) 
of herbaceous plants; and the cullular vessels of the sap-wood (albur- 
num) of trees, that it passes. These vascular cells are little bladders 
replete with juices, a familiar example of the structure of which is 
furnished by the pulp of an orange ; they are arranged side by side, 
but one above another in rather an oblique direction, and, therefore, 
the sap in vessels so constructed and situated, must flow by a lateral 
or zigzag motion, whose general direction is one of ascent. This 
motion depends upon the attractive energy exerted between fluids of 
different densities, when they are seperated by a membranous sub- 
stance ; and constitutes that surprising jihenomenon that has been 
recently discovered, termed by M. Dutrochet, Endosmosis, and by 
some other naturalists, the penetrativeness of fluids. The subject 
would require too much minute enquiry and rigid investigation, to 
admit of being pui'sued in the present paper ; it will, therefore, suffice 
to observe that a species of circulation of the sap may still be said to 
exist, because, to a certain extent, there is a blending of the perfected 
descending juices, with the raw ascending sap. This fact appears to 
have been established by the experiments of Mr. Knight ; for he dis- 
covered that " the specific gravity of the sap of trees increases in the 
spring in proportion to its distance from the ground ; and that sac- 
charine matter, (that is, something in a degree approaching to sugar,) 
is found at that season in the sap-wood of trees, which contained none 
