DIV. I 



MORPHOLOGY 



93 



layers or strands of collenchyma or sclerenchyma ; this is placed as 

 near to the periphery as possible, sometimes lying just below the 

 epidermis of projecting ridges (Fig. Ill, 1, 2). 



RIGIDITY AGAINST BENDING while the least possible mechanical material is 

 employed is best attained by placing this in a peripheral position. "When a 

 straight rod (Fig. 110) is bent the convex side elongates and the concave side 

 is shortened. The outer edges, a, a and a', a', are thus exposed to the greatest 

 variations in length, while nearer the centre (i, i ; i' y i'} the deflection and consequent 

 variations in length are less. If instead of the uniform rod the mechanically 

 effective material were disposed as economically as possible, it should be brought 

 close to the periphery. In this position it will oppose the greatest resistance to 

 bending, and if bending takes place will be less easily torn or crushed than less 

 resistant material. E^ery one knows how great is the resistance to bending of an 

 iron tube, even with thin walls. The builder attains a high level of resistance to 



i i 



FIG. 110. 1. Longitudinal section of an elastic cylinder, before bending (dotted outline) and after 

 bending (heavy outline). After bending the convex side (a') is stretched and the concave side 

 (a) compressed. /, Connecting tissue. 



2. When the connecting tissue (/) is not sufficiently firm, the bands of stereome (a, a') curve 

 independently and remain unaltered in length. (After NOLL.) 



bending by placing at the periphery of structures bars of mechanically effective 

 material parallel to one another and to the longitudinal axis of the structure. 

 These are called girders. It is essential that these GIRDERS should be connected 

 and kept at their proper distances from one another by a sufficiently rigid but 

 elastic connecting tissue (Fig. Ill, 1). Each rod or girder then forms with the one 

 lying immediately opposite an I-girder, the material which occupies the line 

 between the two rods being the connecting material (Fig. 110). Were this 

 connection wanting each rod would be readily bent. In hollow structures, 

 however, it is sufficient that the girders should be joined laterally. In large 

 buildings the peripherally-placed bars have themselves the construction of I- 

 girders, each being constructed of two connected bars. 



As SCHWENDENER ( u ) first showed, the mechanical tissues which render a 

 stem rigid against bending are arranged so as to make the best use of the material. 

 In many plants the mechanical tissue forms a peripheral hollow cylinder which 

 may either come next the epidermis or be more deeply situated (Fig. 112 pc) ; in 

 others there is a system of similarly-placed strands of mechanical tissue lying 

 side by side (system of simple girders, Fig. Ill, 1) ; the latter arrangement may be 

 combined with the complete hollow cylinder (Fig. Ill, 2). In other cases each of 



