PROPULSION OF SAP AND ITS VARIOUS EFFECTS 397 



A distinction is sometimes made between this excretion — 

 due to the special local activity of a certain group of cells — 

 and the somewhat passive excretion of water from water- 

 pores, which is said to be caused by the general pressure of 

 exudation. But this difference is really one of degree and 

 not of kind. All cells are excitable, and the exudation 

 pressure itself is produced by cellular activity. As regards 

 excitability, however, we may have a transition from 

 moderately to highly excitable cells distributed in a continu- 

 ous or discontinuous manner. Even in the stem we have 

 seen that there are cases of irregular distribution, bringing 

 about irregularities of water-pressure. Extreme instances of 

 these are found in Desmodium, and in the actively excreting 

 cells of Colocasia, where highly excitable cells are localised in 

 special areas. A test which is sometimes insisted on as a 

 means of distinguishing between the active and so-called pas- 

 sive excretions is, that in the latter case the flow ceases from 

 the excreting organ, as soon as the branch is cut off. But 

 this is not by any means a satisfactory proof of the absence 

 of active excreting cells in the latter. We saw that the only 

 distinction between the activities of the multiply-responding 

 tissues of Biophytunt and Desmodium, lay in the fact that the 

 latter had the capacity to hold latent a large amount of 

 energy by which the rhythmic activity was maintained even 

 on the cessation of a directly exciting cause. In the case of 

 Desmodium, indeed, if the tonic condition be above par, and 

 the leaf as a whole be cut off and isolated, without any supply 

 of water, the rhythmic activity will be maintained for a con- 

 siderable time, though the turgidity is undergoing constant 

 diminution. But when the tonic condition of the plant is 

 below par, its rhythmic activity comes to a stop, and can 

 only be maintained by an artificial increase of internal 

 hydrostatic pressure. Similarly in the case of water-excreting 

 organs, we have some which under a favourable condition can 

 maintain their activity for a considerable period, even when the 

 supply of water is cut off, while in other instances activity can 

 be maintained only under favourable conditions of turgidity. 



