148 PKOGKKSSION OF THE SAP. 



exosmotic currents in an opposite direction — i.e. downwards and 

 upwards respectively. In summer exosmotic currents flow equally 

 in both directions. These counter-currents are indicated on the dia- 

 gram by darts pointing in a direction opposite to that of the arrows. 

 One other current exists — viz., a lateral current, represented by hori- 

 zontal darts. By this current, sap which has been abstracted from 

 the currents passing along the main channels, is diffused into sur- 

 rounding tissue. Although the upward and downward currents are 

 respectively most vigorous in spring and autumn, stiU at all periods of 

 the year currents of sap pass both upwards, downwards, and transversely. 



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, by which it descends. The cellular tissue is also probably 

 concerned in the movements. Cambium is produced in these plants 

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

 diffused through the textur& 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 ; according to Hoff- 

 mann there is no channel for the descent of fluids in Acrogens, the sap 

 simply ascending and diffusing itself in the substance of the plant in 

 its progress. In cellular plants transmission of the sap takes place 

 from one cell to another ; and as their texture is often delicate, the 

 movements are rapid. Many of these, as seaweeds, when plunged 

 into water, after having been dried by evaporation, imbibe the fluid 

 with very great rapidity. 



The Cause of the Pkogeession or the Sap has been investi- 

 gated by numerous physiologists. While the capillarity of the vessels 

 in the higher plants operates to a certain degree, it would appear 

 that the process of endosmose is that by which the continued imbibition 

 and movement of fluids is chiefly carried on. From the loss of its 

 watery contents, by exhalation, and the metamorphoses going on 

 during the process of nutrition and secretion, the sap becomes 

 gradually more and more dense, and thus throughout the whole 

 plant there is a forcible osmotic transmission of the thinner fluids, 

 and a constant change in the contents of the cells and vessels. These 

 movements will of course take place with greater vigour and rapidity 

 according to the activity of the processes going on in the leaves, 

 which thus tend to keep up the circulation. While the ascending 

 movement of the sap is powerfully promoted by the active operation at 

 the siurface of the leaves, its lateral movements are no less influenced 

 by the individual relations of each distinct cell, since the different func- 

 tions of separate cells, when actively exercised, call into action those vital 

 agencies by which a transmission of the cellular contents is effected. 



