110 
PHYSIOLOGICAL VIEWS 
oftliL' leaf, it enters into the vessels and cells which extend through- 
out its substance. 
The ascending sap is always in circulation, but its energy varies 
with the season, and the age of the plant. Heat has an important in- 
fluence in quickening the ascent of the sap ; yet, during a dny and hot 
season, it often appears to ascend but slowly. This is because the 
absorption of fluids from the earth is checked by the dryness of the 
soil. The plant, by a little stretch of the imagination, may be con- 
sidered as thirsty, and thus man may seem not only provident, but 
humane, in administering to its roots refreshing draughts of water. 
Even the leaves, at such a period, seem too impatient to wait for 
supplies by means of the connecting sap-vessels ; for if water is 
sprinkled upon them, they fail not to use their own power of absorp- 
tion, and upon such an application, may be seen to revive almost in- 
stantaneously. 
When the moisture of the earth coincides with elevation of tem- 
perature, the sap ascends with the greatest rapidity ; this is the case 
in spring. It is at this period, that incisions are made into the wood 
of maple-trees, in order to procure sap for the manufacture of sugar. 
The sap may at this time be seen flowing almost in a stream. It 
has been thought that the circulation of sap was wholly suspended 
during winter; this, however, seems not to be the case; for we 
may observe during this season a gradual development of some 
parts of the plant; we see many plants preserving the freshness 
and verdure of their foliage, and mosses putting forth their flowers 
We must then believe, that the sap is in perpetual motion, suscepti- 
ble of being accelerated or retarded by changes of temperature, and 
humidity, or dryness of the earth. The development of buds must 
be attributed to the ascension and redundancy of the sap, which di- 
lates and nourishes their parts. In spring, when' the ascent of the 
sap is accelerated, the buds enlarge rapidly, and their complete de- 
velopment is soon perfected. 
The vascular texture appears by its tubes and channels to afford 
great facilities for the ascension of the sap. In imperfect plants, such 
as mushrooms and lichens, which are wholly composed of cellular 
texture, it is not known that there is any ascent of sap, but they seeni 
to be nourished by fluids absorbed from the air. 
The question naturally arises, by what force is the sap made to 
ascend, contrary to the laws of gravitation? Some have asserted, 
that this phenomenon was owing to the contraction and dilatation 
of the air, and of the juices of the plant; others have referred it to 
the action of heat ; these two propositions, however, amount to the 
same thing, since heat is the cause of the contraction and dilatation 
referred to. Some ascribe the ascent of the sap to the irritability 
of the vessels, and the energy o f vital power. 
The latter is but a vague and unsatisfactory explanation, since we 
know neither the cause of this irritability, nor in what this vital power 
consists. There is no doubt but the ascent of the sap is, in a degree, 
owing to capillary attraction, assisted by heat. You will lecoliect 
that the vessels containing this fluid, were described as very small 
tubes, no larger than a hair, and, in most cases, much smaller, since 
few are visible to the naked eye. Those who understand something 
of Natural Philosoph y, know that capillary t ubes have the proper ty 
"what etiect hfts drought upon the p-lant 7— Wiiat two circumstances cause (he raj)id 
ascent of the sap 7— Why are incisions made in maple- trees in the spring, rather than 
ut any other period 7— Perpt'tiial motion of sap— Caiise of development cf beds— Vas- 
cular texture unUke the cellular in alTording facilities for the ascension of sap— Ex 
planations of the pauses uf th« ascent of the .sap. 
