272 PHYSIOLOGICAL DEVELOPMENT. 



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 fastis 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' channels 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 ex- 

 tended outer layers and the internal mass. Just that dis- 

 tribution which we find, is the distribution which these me- 

 chanical actions tend to establish. 



As the plant gains in height, and as the mass of its foliage 

 accumulates, the strains thrown upon its axis, and especially 

 the lower part of its axis, rapidly increase. Supposing the 

 forms to remain similar, the strains must increase in the ratio 

 of the cubes of the dimensions ; or even in a somewhat higher 

 ratio. One consequence must be, that the compressions to 

 which the vessels at the lower part of the stem are subject, 

 become greater as fast as the height to which the sap has to 

 be raised becomes greater ; and another consequence must be, 

 that the local exudation of sap produced by the pressure is 

 proportionately augmented. Hence the materials for nutri- 

 tion of the surrounding tissues being there supplied more 

 abundantly, we may expect thickening of the surrounding 

 tissues to show itself there first : in other words, wood 

 will be formed round the vessels of the lower part of 

 the stem. The resulting greater ability of this lower 



