DESCENT OF THE SAP. 825 
in different parts of Great Britain, where the climatal differences 
are not strikingly at variance, or even at the distance of a few 
miles, or in some cases a few yards, frequently vary much as re- 
gards the nature of their peculiar secretions. A striking illus- 
tration of this fact is mentioned by Sir Robert Christison, who 
found that some Umbelliferous plants, as Cicuta virosa, and 
(Enanthe crocata, which are poisonous in most districts of Eng- 
land, are innocuous when grown near Edinburgh. The causes 
of such differences are at present obscure, but the varying con- 
ditions of soil and moisture under which plants are developed 
have doubtless an important influence upon their secretions. 
From a pharmaceutical point of view, so far as the active pro- 
perties of the various medicinal preparations obtained from 
plants are concerned, this modification in their secretions by 
such causes is of much interest, and would amply repay in- 
vestigation ; for it cannot be doubted but that each plant will 
only form its proper secretions when grown under those circum- 
stances which are natural to it, and that consequently any 
change from those conditions will modify in a corresponding 
degree the properties of the plant. Probably here we have an 
explanation, to some extent at least, of the cause of the varying 
strength of the medicinal preparations obtained from the same 
species of plants when grown in different parts of this country, 
or in different soils, &c. 
C. Descent of the Sap.—After the crude sap has been trans- 
formed in the manner already described, it passes from the leaves 
to the stem, probably to the inner bark, and cambium-layer of 
Dicotyledons ; and apparently to the parenchymatous tissues 
generally of the stems of Monocotyledons and Cormophytes. 
lt then descends in the stems of the several kinds of plants as far 
as the root, and in its course affords materials for the develop- 
ment of new tissues and the production of flowers and fruit ; 
and at the same time undergoes further changes owing to 
metabolism, and deposits its various secretions, &c. (page 823). 
Hoffmann in his experiments upon Ferns, however, could not find 
any path by which the elaborated juices descended in the stem. 
That the elaborated sap in Dicotyledons descends through 
the inner bark and cambium-layer is commonly believed, and 
several facts seem to support this belief. Thus, the formation 
of wood is obviously from above downwards, for when a ligature 
is tied tightly round the bark of a Dicotyledonous stem, or more 
especially if a ring of bark be removed, no new wood is pro- 
duced below the ligature or ring, while there will be an in- 
creased development above it, or roots will be produced there. 
Again, it is well known, that by removing a ring of bark froma 
fruit tree, a larger quantity of fruit may be temporarily obtained 
from that tree, owing to the greater amount of nutritive matter 
which then becomes available for the use of the reproductive 
organs (see page 847). Another circumstance which appears 
