THE INNER TISSUES OF PLANTS. 289 



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. 



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 incipient 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 interstitial nutrition being there supplied more abun- 

 dantly, 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 incipient 

 stem. The resulting greater ability of this lower part of the 

 stem to bear strains, renders possible an increase of height; 

 and while after an increase of height the lowest part be- 

 comes still further strained, and still further thickens, the 

 part above it, exposed to like actions, undergoes a like 

 thickening. This induration, while it spreads upwards, also 

 spreads outwards. As fast as the rude cylinder of dense 

 matter formed in this way, begins to inclose the original 

 vessels, it begins to play the part of a resistant mass, which 

 more and more prevents the contained vessels from being 

 squeezed; while between it and the outer layers the greatest 

 compression occurs at each bend. Thus at the same time 

 that the original vessels become useless, the peripheral cells 

 of the developing wood become those which have their liquid 

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