586 APPENDIX C. 



upward movements in others — unless, indeed, there existed descend- 

 ing tubes too wide to admit of much capillary action, which there 

 do not. Moreover, gravitation is clearly inadequate to cause cur- 

 rents 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 experiments on the absorption 

 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, 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 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 direction. And there seems 

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

 more likely to have distributing appliances 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 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 ? 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 this. The resulting competition 



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

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

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

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

 have resumed to a considerable 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 through 

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

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

 vessels. 



