THE INNER TISSUES OF PLANTS. 287 



distributed through the plant, more or less is everywhere 

 being abstracted here by evaporation, here by the unfold- 

 ing of the parts into their typical shapes, here by both. 

 The result is a tension on the contained liquid columns, which 

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

 tension it is which must be regarded as the force that de- 

 termines 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 

 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 

 irrespective of the circumstances of the individual: absence 

 of the appropriate conditions resulting simply in imperfect 

 repetition of the structures. Hence the fact that in trained 

 trees and hothouse shrubs, dense substance is still largely 

 deposited; though not so largely as where the normal me- 

 chanical strains have acted. Hence, too, the fact, that in 

 such plants as the Elephant's-foot or the Welwitschia mira- 

 bilis, which for untold generations can have undergone no 

 oscillations, there is an extensive formation of wood (though 

 not to any considerable height above the ground), in repeti- 

 tion of an ancestral type : natural selection having here main- 

 tained the habit as securing some other advantage than that 

 of support. 



Still, it must be borne in mind that though intermittent 





