278 PHYSIOLOGICAL DEVELOPMENT. 



we do find. On 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, the phloem, 

 and the cortex. But outside of the soft phloem there is 

 frequently another layer of dense tissue now known as the 

 pericyclic fibres, having freq\iently 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 sub- 

 stance is as completely peripheral as the exogenous mode of 

 growth permits. So, too, in general arrangement is it with 

 the ordinary monocotyledonous stem. Different as is here 

 the internal structure, there yet 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 dif- 

 ferent, 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 

 and supporting peltate leaves petioles which are therefore 



