144 PHYSIOLOGICAL VIEWS. 



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 

 and verdure of their foliage ; and mosses putting forth their 

 flowers. We must then believe that the sap is in perpetual mo- 

 tion, susceptible of being accelerated or retarded by changes of 

 temperature, and humidity or dryness of the earth. The devel- 

 opement of buds, must be attributed to the ascension, and re- 

 dundancy of the sap, which dilates and nourishes their parts. 

 In spring, when the ascent of the sap is accelerated, the buds 

 enlarge rapidly, and their complete developement is soon per- 

 fected. 



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 com- 

 posed of cellular texture, it is not known that there is any as- 

 cent 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? Some have as- 

 serted that this phenomenon was owing to the contraction and 

 dictation of the air, and of the juices of the plant ; others have 

 referred it to the action oi'heat; these two propositions, howev. 

 er, amount to the same tiling, since heat is the cause of the con- 

 traction and dilatation referred to. Some ascribe the ascent of 

 the sap, to the irritability of the vessels, and the energy of vital 

 power. 



This is but a vague and unsatisfactory explanation, since we 

 know neither the cause of this irritability, nor in what this vital 

 |K)wer consists. There is no doubt but the ascent of the sap, 

 is, in a degree, owing to capillary* attraction, assisted by heat. 

 You will recollect that the vessels containing this fluid, were 

 described as very small tubes, no larger than a hair, and, in most 



* The terra capillary, is taken from the Latin, capittus, a hair. 



What two circumstances cause the rapid ascent of the sap? Why am in- 

 cisions made in maple trees in the spring, rather than ar.y other period ? Per- 

 petual motion of sap Cause of developement of buds Vascular texture unlike 

 the cellular in affording facilities for the ascension of sap Explanations of the 

 causes of the ascent of sap. 



