288 PHYSIOLOGICAL DEVELOPMENT. 



mechanical strains cannot be assigned as the direct causes 

 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 in- 

 dividuals 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 increas- 

 ing 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 

 by their continuous passage more and more permeable. This 

 establishment of channels is aided by the wind. Each bend 

 produced by it while yet the tissue is undifferentiated, 

 squeezes towards the place of growth and evaporation the 

 liquids that are passing by osmose from cell to cell; and 

 when the lines of movement become defined, each bend helps, 

 by forcing the liquid along these lines, to remove obstructions 

 and make continuous canals. As fast as this transfer of sap 

 is facilitated, so fast is the plant enabled further to raise it- 

 self, and add to its assimilating surfaces ; and so fast do the 

 transverse strains, becoming greater, give more efficient aid. 

 The canals thus formed can be neither in the centre of the 

 rudimentary axis nor at its surface: for at neither of these 



