270 PHYSIOLOGICAL DEVELOPMENT. 



iu the direction of the leaves than in the opposite direction. 

 But when evaporation ceases at night, this will no longer be 

 the case. The sap drawn to the oscillating parts, to supply 

 the place of the exuded sap, must come from the directions 

 of least resistance. A slight breeze will bring it back from 

 the leaves into the gently-swaying twigs, a stronger breeze 

 into the bending branches, a gale into the strained stem and 

 roots — roots in which longitudinal tension produces, in 

 another way, the same effects that transverse tension does in 

 the branches. 



Two possible misinterpretations must be guarded against. 

 It must not be supposed that this force-pump action causes 

 movement of the sap towards one point rather than 

 another : it is simply an aid to its movement. From the 

 stock of sap distributed through the plant, more or less is 

 everywhere being abstracted — here by evaporation ; here by 

 the unfolding of the parts into their typical shapes ; here by 

 both. The result is a tension on the contained liquid columns, 

 that is greatest now in this direction and now in that. This 

 tension it is which must be regarded as the force that 

 determines the current upwards or downwards; and all which 

 the mechanical actions do is to facilitate the transfer to the 

 places of greatest demand. Hence it happens that in a plant 

 prevented from oscillating, but having a typical tendency to 

 assume a certain height and bulk, the demands set up by its 

 unfolding parts will still cause currents ; and there will still 

 be alternate ascents and descents, according as the varying 

 conditions change the direction of greatest demand — the 

 only difference being, that in the absence of oscillations the 

 the growth will be less vigorous. Similarly, it must 



not be supposed that mechanical actions are here alleged to be 

 the sole causes of wood- formation in the individual plant. The 

 tendency of the individual plant to form wood at places where 

 wood has been habitually formed by ancestral plants, is 

 manifestly a cause, and, indeed, the chief cause. In this, as 

 in all other cases, inherited structures repeat themselves 



