Comparative Physiology. 57 S 



standing on pillars of bark and alburnous wood, for experiment, 

 he found the first aj)pearance of descending sap in the pith and 

 the layers of wood near it, before it appeared in the bark, and 

 before the leaves were out. He also found it continue to de- 

 scend in other trees, though imperfectly, when the bark and 

 albui'num to a considerable depth had been removed for ex- 

 periment; thus showing that the whole wood partakes, though 

 imperfectly, in the descent ; the trees getting sickly after this 

 experiment. Professor Thomson says both ascent and descent 

 are assisted by the contractile power of tissue, which is stimulated 

 to action by the sap. The proper vessels by which it descends 

 are, he says, intercellulary passages, and by these also it ascends. 

 Camphor, it is said, has been found to stimulate and increase 

 growth. DeCandolle thought the course of the descending sap 

 by the intercellular passages also ; that it was by open vessels 

 he thought proved by the fact that the under side of horizontal 

 branches is always thickest, which would not be the case if by 

 closed vessels. Sap generally, he asserts, follows the course of 

 fibres either in ascent or descent most readily, though not 

 wholly or always. Contractility, he says, is the cause of all 

 internal force in the circulation of juices. Young incipient 

 ovules, by their contractile force, cause the sap to deviate and 

 be drawn to themselves. It is this also which opens and shuts 

 the valves of stomata, forcing the water out contrary to endos- 

 raose and capillarity. It forces the sap upwards in drooping pistils 

 and pendent branches. It is also this, he says, which causes the 

 circulation in the cells of Chara, &c., and in the latlclferous 

 vessels of Schultz, which resemble cells in texture. Milller says 

 there is an attraction between the blood-vessels and blood in the 

 capillaries, as it sometimes causes accumulation, but does not 

 see how it can cause circulation; he does not take notice of re- 

 pulsion. The force of circulation in the capillaries he thinks 

 wholly dependent on the action of the heart. Circulation in 

 Dlplozoon and other Entozoa, Ehrenberg discovered to be by 

 vibratory cilice, and the same in detached parts. In Planarias, 

 Echinodermata and other low animals, the motion of the blood 

 is, he says, effected by one or more contractile vessels. In 

 Annelida the contractile organs which give rise to the motion 

 of the blood are situated at different points of the circle ; in 

 some the dorsal vessel acts the part of the systemic heart. 

 At page 42. he says, the motions of stamens, leaf-stalks, &c., 

 have too much resemblance to the irritability of muscles, not 

 to be compared with it. At page 43., he says, plants possess 

 irritability, not sensibility, which is consciousness. In the 

 Library of Useful Knoioledge it is said, different portions of 

 the elaborated sap probably descend by different directions, 

 partly by the fibres of the bark, and partly by the cellular por- 

 3d Ser. — 1843, XI. p p 



