RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 761 



It would naturally he expected that in accordance with the various tasks 

 assigned to roots thei-e should be a diöerence in the arrangement of cells and 

 tissues, and that, especially, supporting roots which exhibit the greatest analogy 

 with erect stems, and subterranean roots, which have so much in common with 

 procumbent and subterranean stem-structures, should resemble them in internal 

 structure. Columnar roots cannot really be distinguished from upright stems in 

 their inner construction, and stilt-roots also present an arrangement of cells and 

 vessels which often agrees much better with that of erect stems than of under- 

 ground rhizomes. In Fragrcea obovata, belonging to the ClusiacefB, the cellular 

 structure of the erect stem is only distinguishable from that of its supporting roots 

 by the somewhat stronger development of the medulla and woody portion of the 

 vascular bundles, but otherwise there is no sort of difference. The stilt-roots of 

 the mangrove figured on p. 759 {Rhizophora conjugata) likewise show a stem-like 

 internal structure. In the centre is a thick pith surrounded by numerous, con- 

 ducting bundles, which together form a hollow cylinder, and are accompanied by 

 mechanical tissue; further outwards come the cork, hypoderm, and a strongly- 

 cuticularized epidermis — exactly the same arrangement required in an erect stem 

 as a protection against bending. In these mangroves the strength is even in- 

 creased by a peculiar tissue, viz. by so-called trichoblasts, peculiai-ly interlaced 

 fusiform cells with very thick walls, which are so hard that even the sharpest 

 knife will scarcely cut through them. Though these adult roots are structurally 

 indistinguishable from stems, this is not usually true of them at early stages in 

 their development. When young, and as yet unthickened, these roots, as a rule, 

 possess an internal structure characteristic of roots in general. 



In mangroves and in the earlier mentioned Clusiaceas, the supporting roots are 

 thick and widely spread, and form extensive foundations which entirely replace 

 the comparatively weak trunk, so far as fixing on the substratum is concerned; 

 they need especially to be protected against bending. A resistance to tension 

 scarcely comes under consideration in these plants. It is quite otherwise in plants 

 whose stilt-roots have to support a stem bearing an extensive and richly-leaved 

 crown. The Pandanus figiu-ed on p. 758 may serve as a type of these. As soon 

 as wind sways the massive crown and slender stem bearing it, the roots sup- 

 porting the stem on every side have alternately to resist bending and strain. 

 If the wind blows from the north, the supporting roots springing from the south 

 side experience a longitudinal pressure as the stem inclines to the south, and are 

 pressed and curved down like an arch, while the supporting roots springing fi-om 

 the north side are at the same time subjected to a powerful strain. When the 

 wind sinks, the stem is again brought into its erect normal position by the elasticity 

 of the south roots. The reverse is the case when the wind attacks the crown and 

 stem from the south. This form of stilt-root must therefore be constructed so as 

 to resist strain as well as bending, and accordingly in the aerial roots of Pandanus 

 are formed two cylinders of mechanical tissue, an outer one which is formed by 

 the hard bast of a peripheral ring of vascular bundles, resembling the arrangement 



