MOTION OF THE BLOOD IN THE VESSELS. 543 



&c. Such, for instance, would appear to be the interpretation of the fact, 

 that whilst any variations in the action of the Heart affect the whole system 

 alike, there are many variations in the Circulation, which, heing very limited 

 in their extent, cannot be attributed to such central disturbances, and must 

 therefore be dependent on causes purely local. Of the nature of these in- 

 fluences, and of the mode of their operation, we shall probably arrive at a 

 more correct knowledge, if we examine the phenomena of the Circulation in 

 those beings, in which the moving power is less concentrated than it is in the 

 higher Animals ; for just as we find in the latter, that the development of 

 special absorbent vessels does not exclude the function of absorption from 

 being still performed by the general vascular system ( 675), so may we here 

 be led to perceive, that there is a generally-diffused force, to which alone the 

 Circulation of the nutritious fluid in the lowest organisms is due, and which 

 is not altogether replaced by the special organ of impulsion, that is developed 

 in the centre of the system in the higher. 



712. The ascent of. the sap in Vegetables is probably to be regarded as due, 

 in part, to the vis a tergo occasioned by the action of Endosmose at the 

 roots; and in part, to the demand for fluid, occasioned by the vital processes 

 taking place in the leaves. For if the stem of the Vine, in which the sap is 

 rising, be cut across, the sap will continue to flow for some time from the top 

 of the lower portion ; and its force of ascent may be shown to be very con- 

 siderable, by tying over the cut surface a piece of bladder, which will be 

 speedily burst, or by affixing to it a bent tube, containing a column of mer- 

 cury, which will be raised to the height of forty inches or more. On the 

 other hand, the attractive force of the leaves is shown by the fact, that if the 

 lower end of the upper division be put into water, it will continue to absorb, 

 as long as the vital actions of the leaves are being performed with vigour; 

 but, if the branch be carried into a dark room, the exhalation from the leaves 

 is immediately checked, and absorption is checked also. The influence of 

 the actions at the periphery of the circulating system, in maintaining the flow 

 of fluid towards the part, is further shown by the fact, that, if a shoot of an 

 evergreen species be grafted on a stock of one with deciduous leaves, a con- 

 tinual and unwonted ascent of sap will be kept up in the latter through the 

 winter; this being evidently due to the demand occasioned at its summit. 

 Again, the recommencement of the annual flow of sap in an ordinary tree, has 

 been found to take place in the first instance, not at its roots, but in the 

 neighbourhood of the buds; for their expansion, under the influence of the 

 returning warmth, exhausts the fluid from the vessels of their neighbourhood ; 

 this, again, occasions a demand from below; and thus the motion is gradually 

 propagated to the roots. Now it has been experimentally ascertained, that if 

 a branch of a vine growing in the open air be trained into a hot-house, it may 

 be made to vegetate during the winter, and to draw up fluid through the 

 stems and roots, whose condition has not been changed. It is evident, then, 

 that in Plants, the demand for fluid, in the organs to which it is distributed by 

 the vascular system, is one of the chief forces by which the supply is obtained. 



713. This is still more evidently the case, in regard to the Circulation of 

 nutritious or elaborated sap, which takes place in the under surface of the 

 leaves and in the bark. The object of this movement is not to convey the 

 fluid in a direct line from one point to another (as in the case with the 

 ascending current), but to supply every part with materials for its growth, or 

 for the production of its peculiar secretions. Hence the vessels in which it 

 takes place, form a minutely-anastomosing net-work, instead of consisting ot a 

 system of straight and distinct tubes. Through this net-work, the latex or 

 elaborated sap is seen to move, exactly as does the blood through the capil- 

 lary vessels of animals. The movement takes place, under favourable circum- 



