no PHYSIOLOGICAL VIEWS 



of the ]eaf, it enters into tlie 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 dry 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, wliich are wholly composed 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? Some have asserted, 

 that this phenomenon vras 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 of vital power. 



The latter is but a vague and unsatisfactory explanation, since we 

 know neither the cause of this irritability, norin 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 recollect 

 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 Philosophy, know that capillary tubes have the property 



What effect has drought upon the plant ?— What two circumstances cause the rapid 

 ascent of the sap ?— Why are incisioris made in maple-trees in the spring, rather than 

 at any other period ?— Perpetual motion of sap— Cause of development of beds— Vas- 

 cular texture unlike the cellular in affording facilities for the ascension of sap— Ex 

 planations of the pauses of the ascent of the sap. 



