FLUID PARTS OF VEGETABLES. 
131 
period, to be too impatient to wait for supplies by means of the 
connecting sap vessels ; but if water is sprinkled upon them, 
they fail not to use their own power of absorption, and may 
often be seen upon such an application almost instantaneously 
to revive. 
When the moisture of the earth coincides with elevation of 
temperature, the sap ascends with the greatest rapidity ; 
this is the case in spring. It is at this period, as you no doubt 
are aware, 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 developement of some 
parts of the plant ; we see many plants preserving the freshness 
andverdure of their foliage; and mosses putting forth their flowers. 
We must then believe that the sap is in perpetual motion, sus- 
ceptible of being accelerated or retarded by changes of tempei- 
ature and humidity or dryness of the earth. The developement 
of buds, must be attributed to the ascension, and redundancy 
of the sap which dilates and nourishes their parts. In spring 
when the ascent of the sap is accelerated, the buds enlarge rapidly, 
and then complete developement is soon perfected. 
The vascular texture appears by its tubes and channels to af- 
ford great facilities for the ascension of the sap. In * imperfect 
plants, such as mushrooms and lichens, which are wholly com- 
posed of cellular texture it is not known that there is any ascent 
of sap, but they seem 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 1 Some have as- 
serted that this phenomenon was owing to the contraction and 
dilatation of the air and of the juices of the plant ; some have 
referred it to the action of heat ; and others, to the irritability of 
the vessels and the energy of vital power. There is still ano- 
ther explanation, and one perhaps more philosophical than any 
of the preceding : viz. that the sap rises upwards by the force 
of capillary attraction. You know that the vessels containing 
the sap have been described as very small tubes, no larger than 
a hair,* and in most cases much smaller, since few are visible to 
* The term capillary is derived from the Latin capillus a hair ; meaning 
email, like a hair. 
Influence of heat in the ascending of sap — Sap of maple trees — The peri- 
od in which sap ascends with the greatest rapidity — Perpetual motion of 
sap — Cause of developement of buds — Philosophical explanation of the as- 
cent of sap. 
