THE INNER TISSUES OF PLANTS. 271 



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 Elephants -foot or the Welwitschia 

 mirabilis, 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 

 repetition of an ancestral type : natural selection having 

 here maintained the habit as securing some other advantage 

 than that of support. 



Still, it must be borne in mind that though intermittent 

 mechanical strains cannot be assigned as the direct causes of 

 these internal differentiations in plants that are artificially 

 sheltered or supported, they are assignable as the indirect 

 causes; since the inherited structures, repeated apart from 

 such strains, are themselves interpretable as accumulated 

 results of such strains acting on successive generations of 

 ancestral plants. This will become clear on combining the 

 several threads of the argument and bringing it to a close, 

 which we may now do. 



282. To put the co-operative actions in their actual order, 

 would require us to consider them as working on individuals 

 small modifications that become conspicuous and definite 

 only by inheritance and gradual increase ; but it will aid our 

 comprehension without leading us into error, if we suppose the 

 whole process resumed in a single continuously-existing plant. 



As the plant erects the integrated series of fronds whose 

 united parts form its rudimentary axis, the increasing area 

 of frond-surface exposed to the sun's rays entails an increasing 

 draught upon the liquids contained in the rudimentary 

 axis. The currents of sap so produced, once established along 

 certain lines of cells that offer least resistance, render them 

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