566 



mitting upward movements in others — unless, indeed, there existeiJ 

 descending tubes too wide to admit of much capillary action, which 

 there do not. Moreover, gravitation is clearly madequate to cause 

 currents towards the roots out of branches that droop to the 

 ground. Here the gravitation of the contained liquid columns 

 must nearly balance that of the connected columns in the stem, 

 leaving no appreciable force to cause motion. Nor does there seem 

 much probability in the assumption that the route of the descending 

 sap is through the cambium layer, since expermients on the absorp- 

 tion of dyes prove that simple cellular tissue is a very bad conductor 

 of hquids : their movement through it does not take place with one- 

 fiftieth of the rapidity with which it takes place through vessels.* 



Of course the defence for these hypotheses is, that there must be a 

 downward current, which must have a course and a cause ; and the 

 very natural assumption has been that the course and the cause must 

 be other than those which produce the ascending current. Never- 

 theless there is an alternative supposition to which the foregoing 

 considerations introduce us. It is quite possible for the same vascular 

 system to serve as a channel for movement m opposite directions at 

 different times. We have among animals well-known cases in 

 which the blood-vessels carry a current first in one direction and 

 then, after a brief pause, m the reverse du-ection. And there seems 

 an d, priori probability that, lowly organized as they are, plants are 

 more likely to have distributing appHances of this imperfect kind than 

 to have two sets of channels for two simultaneous currents. If, led 

 by this suspicion, we inquu-e whether among the forces which unite 

 to produce movements of sap, there are any variations or inter- 

 missions capable of determining the currents in different directions, 

 we quickly discover that there are such, and that the hypothesis of 

 an alternating motion of the sap, now centrifugal and now centri- 

 petal, through the same vessels, has good warrant. What are the 

 several forces at work? Fh-st may be set down that tendency 

 existmg in every part of a plant to expand into its typical form, and 

 to absorb nutritive liquids m domg this. The resultmg competition 



* Some exceptions to this occur in plants that have retrograded in tbo 

 charai^ter of their tissues towards the simjjler vegetal types. Certain verj? 

 succulent leaves, such as those of Sempervivum, in which the cellular tissue 

 is immensely developed in comparison with the v^ascular tissue, seem to 

 have resumed to a considf^rable extent what we must regard as the primitive 

 form of vegetal circulation — simple absorption from cell to cell. These, 

 when they have lost much of their water, will take up the dye to some dis- 

 tance through their general substance, or rather through its interstices, even 

 neglecting the vessels. At other times, in the same leaves, the vessels will 

 become charged while comparatively little absorption takes place througli 

 the cellular tissue. Even in these exceptional cases, however, the movement 

 through cellular tissue is nothing like as fast as the movement through 

 vessels. 



