566 



mitting upward movements in others — unless, indeed, thore esisteJ 

 descending tubes too wide to admit of mucli capillary action, wliicli 

 there do not. Moreover, gravitation is clearly uiadequate 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 seeiu 

 much probability in the assumption that the route of the descending 

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

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

 of liquids : 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, wMch 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 in 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, in the reverse dii'ection. And there seems 

 an ct priori probabUity 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 inquire whether among the forces which unite 

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

 missions capable of determining the cm'rents 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? First may be set down that tendency 

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

 to absorb nutritive liquids in doing tins. The resulting competition 



* Some exceptions to tliis occur in plants that have retrograded in tho 

 character of their tissues towards the simpler vegetal types. Certain very 

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

 is immensely developed in comparison with tlie vascular tissue, seem to 

 nave resumed to a oonsidi^able extent what we must regard as the primitive 

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

 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, eveii 

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

 become charged while comparatively little absorption takes place through 

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

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

 vesaele. 



