TISSUES OF fl.ANTS. 261 



dense substance able to resist these tensions and pressures is 

 deposited most where they are greatest, we ought to find it 

 taking the shape of a cylindrical casing. This is just what 

 we do find. Oil cutting across a shoot in course of formation, 

 we see its central space either unoccupied or occupied only 

 by soft tissue. That the layer of hard tissue surrounding 

 this is not the outermost layer, is true : there lies beyond it 

 the cambium layer, from which it is formed. But outside 

 of the cambium there is another layer of dense tissue, the 

 liber, having frequently a tenacity greater even than that of 

 the wood a layer which, while it protects the cambium and 

 offers additional resistance to the transverse strain, admits of 

 being fissured as fast as the cylinder of wood thickens. That 

 is to say, the deposit of resisting substance is as completely 

 peripheral as the exogenous mode of growth permits. So, 

 too, in general arrangement is it with the endogenous stem. 

 Different as is here the mode of growth, and different as is 

 the internal structure, there } r et holds the same general dis- 

 tribution of tissues, answering to the same mechanical con- 

 ditions. The vascular woody bundles, more abundant towards 

 the outside of the stem than near the centre, produce a harder 

 casing surrounding a softer core. In the supporting 



structures of leaves we find significant deviations from this 

 arrangement. While axes are on the average exposed to 

 equal strains on all sides, most leaves, spreading out their 

 surfaces horizontally, have their petioles subject to strains 

 that are not alike in all directions ; and in them the hard 

 tissue is differently arranged. Its transverse section is 

 not ring-shaped but crescent-shaped : the two horns being 

 directed towards the upper surface of the petiole. That this 

 arrangement is one which answers to the mechanical con- 

 ditions, is not easy to demonstrate : we must satisfy ourselves 

 by noting that here, where the distribution of forces is 

 different, the distribution of resisting tissue is different. And 

 then, showing conclusively the connexion between these differ- 

 ences, we have the fact that in petioles growing vertically 



