QO LECTURE VI. 



These changes are more especially active for different sub- 

 stances in different parts of the plant, and consequently the 

 rapidity of transmission of certain substances in any par- 

 ticular direction will be greater than that of others. Thus, 

 the inorganic substances absorbed by the roots pass into the 

 cells of the leaves where they are concerned in the processes 

 of constructive metabolism which are in operation in those 

 organs, and the products of these processes pass from the 

 leaves, either to those parts of the plant in which growth 

 is actively proceeding and plastic material is required, or to 

 those parts, such as seeds, tubers, etc., in which stores of 

 organic substances are being laid up. 



This is the only way in which the distribution of water is 

 effected in cellular plants, in plants, that is, which do not 

 possess fibrovascular bundles. It is only one of the ways in 

 vascular plants, and we will now endeavour to ascertain what 

 the other ways are. 



It is a matter of common observation, which Ray seems 

 to have first carefully noted, that when the stems of vascular 

 plants are cut in the spring, a flow of watery fluid frequently 

 takes place from the cut surface of that portion of the stem 

 which is connected with the root. This fact was investigated 

 with great thoroughness by Hales, and he concluded that 

 there is "a considerable energy in the root to push up sap in 

 the bleeding-season." The correctness of Hales' conclusion 

 was confirmed by the experiments of Dutrochet, in the course 

 of which he ascertained that a flow of sap only takes place 

 from the cut surface of a stem when it is organically con- 

 nected with the roots, and from that of a root when it is in 

 connexion with the soil by means of root-hairs. The Root- 

 pressure, as this force is termed, is therefore the expression of 

 the absorbent activity of the root-hairs. 



This subject was studied at a later period by Hofmeister. 

 He found that " bleeding " is not peculiar to woody plants, as 

 his predecessors had supposed, but that it occurs in herba- 

 ceous plants also; and further, that, although it can be most 

 readily observed in the spring, it may be artificially induced 

 at any season. 



