762 RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 



occurring in the majority of dicotyledons, and an inner which is formed of the 

 hard bast of a ring of vascular bundles lying near the centre of the root. By the 

 former the supporting roots are afforded the necessary resistance to bending, and 

 by the latter the corresponding resistance to strain. 



The stilt-roots springing from the lowest nodes of maize -plants are adapted 

 to this double function just as are those of Pandanus. Here also are two cylinders 

 of mechanical tissue. The outer one, situated in the cortex, consists merely of 

 hard bast and provides a resistance to bending, while the inner, in connection with 

 the conducting bundles, furnishes a resistance to strain. In the stilt-roots at the 

 base of the maize-stem there is, however, a central pith or wide medullary cavity 

 which is wanting in the roots of Pandanus. 



Clinging roots adhering to the bark of trees, stones, or some other hard sub- 

 stratum, as well as the many forms of subterranean roots, are not required to resist 

 bending, and in them there is none of the mechanical tissue which would be 

 necessary for this resistance. On the other hand, these roots are unavoidably 

 subject to a severe strain from the pulling exerted by the stem and branches as 

 they sway to and fro. For a cylindrical body which has to resist a powerful 

 longitudinal strain there is no better contrivance than the fusion of the resisting 

 elements into a compact mass in the axis of the cylinder, and this arrangement is 

 actually met with in clinging and subterranean roots. The conducting bundles 

 and the adjoining mechanical tissue form a single central strand in the cylindrical 

 root, and the typical form of a subterranean root is a cylindrical body of tissue 

 which has no central pith and no hard bast cylinder near the circumference, but 

 whose vascular bundles are so crowded towards the axis that they form there a 

 single, thick strand or "central cylinder". 



Roots embedded in the ground are of course exposed to a lateral pressure from 

 their surroundings, and care must be taken that the functions of the conducting 

 bundles are not disturbed by this pressure, that the transmission is not interrupted 

 or even entirely stopped. This is effected by padding the central strand just 

 described, that is, by surrounding it with a mantle of parenchymatous cells. The 

 thickness of this coat varies according to the extent of the lateral pressure, and 

 when the roots are subjected to very great pressure, the walls of the parenchyma- 

 tous cells are even thickened in a corresponding degree. 



Reserve-materials may also be deposited in this parenchymatous mantle. In 

 biennial and perennial roots the tissue surrounding the sap-conducting and strain- 

 resisting strand is not only thickened so as to give the necessary support against 

 pressure, but also provides a place for starch, fat, sugar and other supplies which 

 are to be consumed in the next period of vegetation. 



Naturally these soft tissues, often filled with reserve-food, are an attraction to 

 diverse animals living underground, and the establishment of such a storehouse 

 renders a corresponding protection against the attacks of mice and insect-larvae 

 necessary. Though the protective agents and weapons by which the green foliage, 

 and flowers, and fruit are preserved from the ravages of animals would not serve 



