102 ASCENT OF THE SAP. ENDOGENOUS STEMS. 



the substance of the roots, down which it is conveyed by their 

 bark. In this descent, it mixes with the ascending current, 

 especially at its lower part ; and being much superior in density, 

 it adds to the density of that fluid, and thus maintains the con- 

 ditions requisite for endosmose (. 119). The 

 vessels down which the sap moves in the bark, 

 are of the branching character, described as 

 peculiar to those which convey the nutritious 

 fluid (. 87). They form a complete network, 

 in which the fluid may be seen to move in 

 various directions. For this motion no definite 

 cause can be assigned. It does not depend on 

 any impulse from above, corresponding to that 

 Fro. 51,-BRANCHiNG action of the roots which raises sap in the 

 VESSELS OF THE BARK, stem ; for there is no power in the leaves to 

 give any such force. It has been supposed to depend upon the 

 gravity of the fluid, which will cause it to descend simply by its 

 own weight ; but if that were the case, it would not ascend, as it 

 often does, in the bark of the hanging branches of such trees as 

 the Weeping Ash or Willow. It is only one, however, of 

 numerous cases in which a movement of nutritious fluid, through 

 channels in the solid parts it supplies, takes place without any- 

 evident cause, in Animals as well as Vegetables. (See ANIM. 

 PHYSIOL. . 280.) 



141. The stem of Endogens is formed upon a very different 

 model. As already stated, the woody bundles in the stem of a 

 year's growth, such as that of the Asparagus, are distributed 

 through the whole of the cellular mass, which originally consti- 

 tuted it ; and a similar arrangement would be found in the stem 

 of a Palm, or, other aged tree. Instead of being united into 

 rings, these bundles remain separate ; and it is only on the 

 exterior of the tree, where they are closely pressed together, in 

 consequence of the continual addition of new woody matter to 

 the interior, that they form anything like hard wood ; and even 

 this, though very useful for some purposes, does not possess the 

 kind of texture which adapts it to the work of the artisan. 

 Each annual set of woody bundles, proceeding (as in Exogens) 



