FLUID PARTS OF VEGETABLES. 105 



a streani. 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 ; Ave see 

 many plants preserving the freshness and verdure of their foliage, and mosses put- 

 ting 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 development 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 

 their complete development is soon perfected. 



120. The theory of the movements which take place between 

 fluids of different densities, called Endosmosis and Exosmosis^ 

 01 imhihition and penneahilitj/, explains many of the phenom- 

 ena in vegetable physiology. It is found that if two fluids oi 

 different density be separated by a membrane without visible 

 pores, the lighter fluid is imbibed by the membrane ; the denser 

 fluid also passes outward ; thus the two fluids undergo the same 

 process of transudation : the name of Endosmose^ to pass with- 

 in, and Exosmose^ to pass without, has been given to these 

 changes. This process may account for the fact, that about the 

 roots of plants their peculiar qualities may be detected in the 

 soil; and the same is observed of water in which a plant has 

 been made to vegetate. The cell containing a fluid thicker than 

 water, imhihes water by endosmosis^ and yields by exosmosis a 

 portion of its liquid contents to a contiguous cell containing 

 a still denser fluid. When the leaf-buds become stimulated 

 by an increase of sap, the Endosmotic process commences in 

 their cells, and gradually extends to the roots, which are ab- 

 sorbing new supplies of fluid from the soil. The juices which 

 had been accumulated during winter j)ass through changes, cer- 

 tain substances are dissolved, and the sap becomes thickened ; 

 thus the endosmotic process is greatly increased, and an active 

 circulation goes on through the whole system of the plant. 

 Toward the latter part of the season light and heat are less 

 powerful, the leaves perform their office more languidly, and 

 there is more equilibrium in the density of fluids, until circula- 

 tion ceases. 



121. The vascular texture appears by its tubes and channels to afford great fa- 

 cilities 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 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 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. 

 Soniu ascribe the ascent of the sap to the irritability of the vessels, and the energy 

 oi vital power. The latter is but a vague and unsatisfactory explanation, since we 



loT^Tr^'"*^ motion of sap— Cause of development of buds.— 120. Endosmosis and Exosmosis.— 

 1~1 V ascu.ar texture unlike the cellular in aflording facilities for the ascension of gap— Explanations 

 ol t!»e eauses of the asoent of the ban. 



