LIFE OF THE WHOLE PLANT. 781 



(p. 726), and therefore in Dicotyledons when they are of any age, 

 through the outer portion of the wood or the alburnum. In such 

 plants also, we have but one main stream of ascending sap. In 

 Monocotyledons and Acotyledonous stems, the ascent also takes 

 place through the unincrusted cells of the fibro- vascular bundles; 

 and hence in such plants, and more especially in Monocotyledons, 

 we have a number of more or less distinct ascending streams. In 

 the lower Acotyledons, as Thallogens, which have no stems, there 

 is no regular course of the sap, but the fluids may be noticed 

 flowing in all du-ections through their cells, and to be more 

 especially evident in those parts which are of a lax nature. 



The cause of the flow of the sap in Thallogens and in plants 

 even such as Mosses, &c., which have no fibro-vascular tissues, is 

 simply endosmotic action, arising from the varying densities of 

 the contents of the cells at difi"erent parts. 



In the higher plants, such as those which have true stems with 

 fibro-vascular bundles, the principal cause of the ascent is, also, 

 endosmotic action. This is probably assisted also, at certain 

 periods, such as in the spring of the year, &c., when the whole 

 plant is gorged with sap, by capillary attraction. (Trecul says 

 that capillary attraction and endosmose are quite insuflicient to 

 account for this rise of the sap. He has also put forward some 

 entirely new views regarding the distribution of fluids through 

 the plant, but as these have not been confirmed we cannot 

 explain them here.) The mode in which endosmotic action 

 thus produces the ascent of the sap may be explained as fol- 

 lows : — The fluid contents of the cells of the leaves and other 

 external parts where transpiration or assimilation is going on, 

 become increased in density, and thus act by endosmotic action 

 upon the more fluid contents of the cells within them ; these 

 in like manner by becoming thickened, also react upon those 

 below them, which in like way do so upon those beneath, and 

 so on, until the influence is communicated to the cells of the 

 root by which fluid is absorbed from Avithout, and thus the in- 

 creased density of the fluid contents of the leaves, &c., is the 

 cause of endosmotic action, and this leads directly to the ascent 

 of the sap through the stem. This influence of the leaves in 

 promoting the absorption and ascent of fluid matters is well 

 shown by introducing a branch of a plant growing in the open 

 air in winter, into a hot-house. In this case, as soon as the 

 leaves expand, absorption and ascent of fluid matters imme- 

 diately take place. 



Endosmose being thus shown to be the principal direct cause 

 of the ascent of the sap, the force with which this ascent takes 

 place will necessarily vary at the ditiSerent seasons of the year, 

 according to the activity of those processes, such as transpiration 

 and assimilation, by which condensation of the fluid contents of 

 the leaves, &c., is produced. Thus in the winter no transpira- 



