Physiological Functions of Plant Tissues. 265 



flat and ribbon-like. So far as regards rigidity, there is 

 generally a tendency of the bast to the periphery. 



But there is another tendency of the wind hesides 

 bending, viz., dragging or tearing a tree out by its roots. 

 And if the bast is at the periphery, any slight irregularity 

 might lead to the breaking of a single string. Hence there 

 is also a development of the bast in the centre. This is 

 especially seen in roots and rhizomes, and also in the 

 tendrils and stems of climbing plants. This arrangement is 

 also the best for supporting the longitudinal pressure from 

 above, due to the weight of the tree. 



In Pandanus odoratissimus and Zca Mays a circle of 

 adventitious roots grow out round the base, which act both 

 as buttresses and anchoring fibres. 



A cross section of these roots near the ground shows a 

 strong central development of bast, and an external sub- 

 epidermal hollow cylinder. Perhaps it may be interesting 

 to note, that in the Forth Bridge the girders starting the 

 arches, and which have exactly the same object to fulfil, 

 have the same arrangement of a central mass within a 

 hollow cylinder. 



A leaf blade is also exposed to shearing stresses, but these 

 are resisted by the anastomoses of the veins. Sometimes a 

 system of connected veins runs right round the edge, 

 especially in serrate leaves, and a transverse semicircular 

 development of bast round the edge, for the same purpose, 

 is found in many plants. 



The mechanical system usually develops from the 

 cambium. If there are separate individual cambium 

 bundles, one bundle may form, as in Juncus glmicus, both 

 the hard bast of the exterior and interior part of the 

 mechanical system, as well as the central vaso-parenchyma- 

 tous portion between the two. So also in Urabellifera?, both 

 the oil canals and the mechanical bast ribs arise from a 

 common cambium string. 



In the case of the scattering of Geranium seeds, and also 

 in the leaves of certain steppe grasses, movements are 

 produced by the bast cells. In the latter case, the outer 

 bast of the leaf consists of two layers. In very dry weather 

 the innermost of these layers contracts, causing the leaves to 

 roll up, and they uncurl when it becomes moister. There are 



