574 Compm-ative Physiologtf. 



tion of it, and along the medullary rays, the force which carries 

 along this last not being defined. 



Gravity and vibration are causes of descent that are obvious 

 and easily understood; vibration probably assists, as I before 

 noticed, in the removal of unsound particles, and may help in 

 this way to consolidate. Plants exposed to the action of 

 moderate winds are always more firm, healthy, and vigorous than 

 those confined. The assistance of gravitation in descent is seen 

 in the fact that all branches have their vigour much lessened 

 when depressed artificially in training, and the tendency of the 

 branch to start a shoot in the vertical direction from the 

 upper part of the bend. Most writers seem to acknowledge 

 the necessity of an internal vital force ; and the most ob- 

 vious and most generally referred to is that of a contractility, 

 similar, though of a lower kind, to the contractility of the heart. 

 Dr. Carpenter does not say how it is certain that it is inde- 

 pendent of contractility of vessels. DeCandoUe says that, 

 though general through all the tissues, the contractile force 

 principally resides in the cells. The force, which he describes, 

 of attraction between the nutriment and the walls of the vessels 

 or cells through which it moves, is generally allowed, but can 

 be no more explained than other vital actions. That of repul- 

 sion has not been so generally allowed ; and how the same sub- 

 stances that once attracted will repulse, cannot be understood 

 unless from some chemical changes giving rise to opposite states 

 of electricity. It is noticed that a current of electricity, con- 

 ducted along the laticiferous tubes, stops the circulation. The 

 power of contractility has been said to be elicited by electricity ; 

 and the two powers may be the same under different names. The 

 course of the downward flow seems most likely to be through the 

 intercellular passages, and most forcibly along the fibro- vascular 

 portion of the bark, and along the fibres and ducts of the liber, 

 as along those of the alburnum in ascending. It is highly pro- 

 bable also that networks of capillary vessels or tubes exist, and 

 may assist in carrying off waste as well as depositing. The 

 oxygen of the air may be united to insufficient particles in the 

 young soft wood, or the newly deposited layers of the old ; and 

 they may be absorbed and carried off by vessels similar to the 

 laticiferous, but would require to be situated in, or connected 

 with, the alburnum as well as bark. The constant extrication 

 of carbonic acid from the stomata would seem to infer a neces- 

 sity for something of the kind ; and the circumstance of unsound 

 portions being seldom found in healthy well-ripened young 

 wood, and being sometimes found partly to disappear from very 

 young shoots by the further growth of the same season, is also 

 favourable to the opinion. If shoots kept from moving were 

 found to give off less carbonic acid than those kept in a vibra- 



