202 



MECHANICAL SYSTEM 



definite points in the cross-section and extends gradually from these 

 to the other portions. In other words differentiation is simultaneous 

 in a fibrous strand, but successive in a vascular bundle. 



2. Development of stereome from protoderm. 



As was pointed out in a previous section of this chapter (IV. A. 1), 

 the centrifugal tendency of the mechanical system in inflexible organs 

 often brings the fibrous or collenchymatous tissue into immediate 

 contact with the epidermis. It might be anticipated that this close 



Fig. 74. 



Small portion of a T.S. through the lamina of Pteris tevrata, passing through 

 one of the I-girders ; each flange consists in cross-section of a single bast-fibre 

 of protodermal origin. After Kraus. 



proximity of the mechanical system would not be without its effect 

 upon the structure of the epidermis itself. As a matter of fact the 

 epidermis is, in these circumstances, employed to supplement the sub- 

 epidermal bast or collenchyma in a degree which varies within wide 

 limits. In extreme cases the superficial layer entirely relinquishes its 

 dermal character and becomes completely converted into mechanical 

 tissue. Even where there is no such close approximation of the 

 skeletal and dermal systems, the epidermis may be pressed into 

 tbe service of the former under the dominating influence of the 

 mechanical principle. For it must be remembered that in inflexible 



