56 Vegetable Organography and Physiology. 



can only glance briefly at the course of the sap, and notice the 

 changes which take place in this fluid, in its circulation through 

 the vegetable system. 



De Candolle discovered that the roots of exogenous plants were 

 terminated by small organic bodies, composed of cellular tissue and 

 woody fibre, enclosing in their centre a minute bundle of ducts. 

 These bodies, which are termed spongioles, possess the power of 

 absorbing fluids from the earth with wonderful rapidity. This 

 power has been denominated endosmose, by Dutrochet, who dis- 

 covered, by some ingenious experiments, that the accumulation of 

 a fluid in organic cavities, like the cells of plants, imparted to 

 those bodies a vital principle by which their cavities were alter- 

 nately emptied and replenished. It is a law of this " physico-or- 

 gaiiic action" that the denser fluid always attracts the rarer ; and 

 as the cells of plants are always filled with a matter of a denser 

 consistence than water, this fluid, which always surrounds the 

 roots of trees, is drawn up by the spongioles with great rapidity. 

 This action possesses the property of rendering turgid the con- 

 tents of these cavities, when once imbibed, so that a constant en- 

 dosmose is kept up. Professor Daubeny, by some curious expe- 

 riments, demonstrated that the roots of plants possess the power 

 of selecting such materials as are required for their nourishment 

 and growth, and of rejecting matter whose introduction into their 

 tissues would prove deleterious to the plant. In almost all plants, 

 however, the ascending sap is found to be nearly uniform in its 

 composition ; consisting of water, holding in solution various 

 mineral substances, with atmospheric air. Having once entered 

 the spongioles, the sap is conveyed along in its vertical current 

 from one vesicle to another by this e?ic?oswiowie/nc property of the 

 tissues of the plant, until it arrives at the leaves, where by its 

 exposure to the atmospheric air it undergoes a chemical change, 

 which prepares it to afford vitaUty and nourishment to the plant. 

 This process is considered by physiologists as strictly analagous 

 to the respiration of animals. 



The phenomena connected with the respiration of plants, or 

 the changes produced upon atmospheric air by vegetables, are 

 highly curious and interesting. It was ascertained many years 

 ago, by Bonnet, Priestley, and others, that healthy plants exposed 

 to solar hght, were constantly evolving oxygen gas. But the 



