206 



PHYSIOLOGICAL BOTANY. 



which influence the flow. It is observed that the de- 

 scending current is generally denser and more saccharine 

 than the ascending, although the reverse is occasionally 

 the case after violent rains. Light appears to be the 

 principal agent in modifying the conditions of the flow. 

 Mild weather promotes the ascent, and a sudden cold 

 succeeding causes a rapid descent by contracting the 

 trunk of the tree. If the cold continue and the ground 

 become frozen, the sap is again forced to ascend. When 

 a thaw succeeds a frost the exhausted roots are to be 

 replenished, and the downward current is re-established. 

 The rapid ascent which commences in spring when the 

 buds are beginning to burst, ceases as soon as the leaves 

 are completely expanded. After midsummer the power 

 of the solar rays being less energetic, and the deposition 

 of earthy particles having obstructed the vessels of the 

 leaf less sap is exhaled from them and the tree attains 

 a state of plethory, indicated by an increasing flow at 

 the upper tube of the instrument. 



(192.) Cause of Progression. Although these ex- 

 periments of M. Biot clearly indicate that there is an 

 influence produced by a change of temperature and 

 probably also by other atmospheric causes on the pro- 

 gression of the sap, it is 'neither to these nor yet to the 

 effects of gravity that we must entirely attribute the 

 descent and general diffusion of the nutritious jukes. 

 We find that if a branch is ringed and its extremity 

 bent towards the ground, the tumour now is produced 

 upon that edge which is the lowest in position though 

 furthest from the root, and consequently the return- 

 ing sap has been compelled to rise into the pendent 

 branch. Its progression is decidedly facilitated by 

 mechanical causes, such as the wind continually agitat- 

 ing the stem and branches. Mr. Knight confined the 

 stem of a tree so that it could vibrate only in one 

 plane ; and at the end of some years lie observed that 

 its section was an ellipse, whose greater axis lay in this 

 plane. 



(193.) Intercellular Rotation. In the ascent, 



