§ 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 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 osmosis 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 itself, 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 places can the transverse strains produce any considerable compressions. They must arise along a tract between the outside of the axis and its core--a tract along which there occur the severest squeezes between the stretched outer layers and the internal mass. Just that distribution which we find, is the distribution which these mechanical actions tend to establish.