CYLINDRICAL INFLEXIBLE ORGANS 



177 



tissue (Fig. 62). Examples of typical intra-cambial mechanical cylinders 

 are furnished by the stems of Tropaeolnni majus, Impatiens nolitangere, 

 Centranthus ruber, Sedum reflexum, etc. 



In a purely mechanical sense it is permissible to regard the con- 

 tinuous bast- or wood-fibre-cylinder as the product of the lateral 

 coalescence of a circle of closely crowded fibrous Manges ; but it would 

 be a mistake to assume that the ontogenetic development of the 

 bast-cylinder actually involves any such process of fusion. There is no 

 reason to suppose that any stereome-cylinder has arisen in this way, 

 either in the ontogenv of the individual or during the evolution of a 

 species. As Schwendener has pointed out, the bast-cylinder is " an 

 independent structure, the shape and position of which are not 



Fig. 62. 



Mechanical system of the stem of Aslrantia major. A subcortical fibrous cylinder, 

 with embedded leptome- and internally apposed hadrome-strands is reinforced by 

 subepidermal girders of collenchyma. 



appreciably affected by other tissues. Its structure and arrangement 

 are entirely determined by the mechanical principle. It is not in the 

 least subject to the influence of the peripheral vascular bundles; on 

 the contrary, it is the latter that adapt themselves to the peculiarities 

 of the mechanical cylinder, apposing themselves to its outer or inner 

 face, or becoming completely embedded in its interior, for the sake of 

 the shelter or support that it affords " (Fig. 61a, b). 



The distance of the fibrous cylinder from the surface of the stem 

 rarely exceeds one-twentieth, and may be as little as one-fiftieth, of the 

 total diameter of the organ. On its cortical side it is almost always 

 very thick-walled and hence as a rule sharply differentiated. On its 

 inner side, on the contrary, it often passes insensibly into the 

 medullary parenchyma, the cells becoming gradually shorter, wider 

 and less thick-walled, while the cross- walls at the same time approximate 

 more and more to the strictly transverse position. 



M 



