Coynparative Physiology. 569 



majus^ or the interior sepals of Calystegia sepium. It seems 

 to take place in all directions, the currents often contrariwise in 

 contiguous vessels. Sometimes one of the currents oscillates and 

 stops, and either the same is recommenced or a new current in 

 the same direction. It is owing to this variety in the course of 

 the streams, that, if a stem containing milky juice be cut across 

 in two places, the latter will flow out from both ends of the 

 jjiece so isolated ; and, as the same takes place in species which 

 have a transparent proper juice, it is reasonable to infer that a 

 similar circulation takes place in them. 



" The cause of the ascent of the sap has long been disputed, 

 some attributing it to mechanical, and others to vital inex- 

 plicable, influences. The endosmose of the roots is a partial, 

 but not the entire, cause of the ascent, since it only continues 

 provided the functions of the leaves occasion a demand for it. 

 The upward flow of sap in the spring begins near the buds, and 

 may be progressively observed to extend to the branches, trunk, 

 and roots, the latter not commencing their action until the su- 

 perincumbent column has been removed. The demand for 

 fluid occasioned by the vital processes of the leaves is the cause 

 of the ascent, the propulsive power of the roots raising it to the 

 power of that influence. The cause of the descent cannot be 

 distinctly ascertained ; it has been supposed partly due to gravity, 

 but, though alFected by it, cannot depend on it, as it takes place 

 in pendent branches and bent stems. It is assisted also by vi- 

 brations of the wind. The descending may be called vital cir- 

 culation to distinguish it from the ascending movement of crude 

 sap. It is quite certain that it is independent of any contraction 

 of vessels, and that it is closely connected with the activity of 

 the nutritive principles. The analogy of the cyclosis of Schultz 

 with the capillary circulation of animals has been already noticed ; 

 the latex is most abundant in parts in progress of developement, 

 and the movement is much influenced by temperature. The 

 cause of the movement appears to be a new set of attractions 

 and repulsions, created between the particles of the fluid and the 

 walls of the vessels through which they move, by the changes to 

 which both are subjected in the process of nutrition. The 

 obvious independence of the cyclosis on any thing like a central 

 organ of impulsion supports the belief that the capillary circu- 

 lation is maintained in the lower animals, and modified in the 

 higher, by influences originating in itself, and is never entirely 

 dependent on the action of the heart. 



In the embryo the absorption takes place through the whole 

 surface ; there is no transmission of fluid, nor any vascular struc- 

 ture ; and it is therefore on a level, as to circulation, with the 

 simplest cellular tribes. It is not until the true leaves are ex- 

 panded that we find a distinct formation of woody or vascular 



