128 CIRCULATION OF THE SAP. 



marked oscillation in the vessels showing their continuity, there could, 

 nevertheless, be observed a regular movement from the apex towards 

 the base, independent of external influences, when the stipule was 

 simply allowed to lie on the field of the microscope without any pres- 

 sure or injury whatever. This movement continued for at least twenty 

 minutes during one of the experiments, and I have no doubt might have 

 been observed longer. It is of importance to distinguish between 

 those molecular movements which are caused by injury and pressure, 

 and those which depend on processes going on in the interior of the 

 living plant. My experiments are by no means complete, but they 

 lead at present to the adoption of Schultz's opinion relative to the 

 existence of cyclosis. 



263. The elaborated sap descends partly by the vessels of latex, and 

 partly by those of the liber. It has been said that there is sometimes 

 a difference in the sap contained in these two kinds of vessels. Occasion- 

 ally, as in Euphorbia canariensis, the elaborated sap has acrid properties, 

 while the ascending sap is bland and wholesome. The elaborated sap 

 contributes to the formation of the cambium, which is produced be- 

 tween the bark and wood of exogens. 



264. It appears, then, that in the case of Exogenous plants, the 

 fluid matter in the soil, containing different substances in solution, is 

 absorbed by the extremities of the roots, ascends to the stem, passes 

 through the woody tissue, porous vessels and cells, dissolving and ap- 

 propriating various new substances. Proceeding upwards and out- 

 wards, this sap reaches the leaves and the bark, where it is exposed to 

 the air, and is elaborated by the function of respiration. It then 

 returns, or descends chiefly through the bark, either directly or in a 

 circuitous manner, communicating with the central parts by the 

 medullary rays, depositing various secretions, more especially in the 

 bark, and giving origin to substances which are destined to nourish 

 and form new tissues. Finally, it reaches the extremity of the root, 

 where absorption had commenced ; a small portion is there excreted, 

 while the remainder mixes with the newly-absorbed fluids, and again 

 circulates in the sap. 



265. In the case of Endogenous plants, observations are still wanting 

 by which to determine the exact course of their fluids. The vascular 

 bundles contain woody vessels, which probably are concerned in the 

 ascent of the sap, and vessels equivalent to those of the bark and of the 

 latex, which serve for the descent of it. The cellular tissue is also proba- 

 bly concerned hi the movements. Cambium is produced in these plants 

 in the neighbourhood of the vascular bundles, and is thus generally 

 diffused through the texture of the stem. In acrogenous stems, it is 

 likely that the sap follows the same course as in Endogens, although, 

 in regard to both, experiments are still wanting. In cellular plants, 

 transmission of the sap takes place from one cell to another ; and, as 



