194 PHYSIOLOGICAL CLASSIFICATION OF TISSUES. 



in determining the actual force exerted upon the fibres at the 

 base of the branch. 1 



534. The part which sclerotic parenchyma and thickened 

 epidermal and hypodermal cells play in affording strength to 

 plants need only be alluded to (see 211). In a few cases, 

 especially in some succulents, a considerable share of the me- 

 chanical support of the plant is afforded by the more superficial 

 parts. 2 



535. The reining of leaves and the structure of leaf-margins 

 present some interesting problems. Comparative investigations 3 

 have shown that strength at the edge of the leaf is obtained in 

 very different wa} T s, even in closely allied plants. The resist- 

 ance to tearing which is exhibited lry some of the larger leaves 

 of dicotyledons is remarkable. 



The distribution of the strong ribs in the leaves of the greater 

 water lilies (for instance, Victoria regia), and to a less striking 

 extent that in the smaller water lilies of cold climates, secures 

 great strength with the utmost economy of material. 



The trunks of man}* tropical trees are provided with lateral pro- 

 jections (buttresses) which strengthen the stem very materially. 4 



veinletg come off from the midrib is the same as that formed by the branch and 

 the trunk. The angles in the above cases are those formed above the points 

 where the branches arise (British Assoc. Report, 1852, part ii. p. 68). 



1 Very instructive illustrations of the different capacity of different trees to 

 resist the action of high winds are given in the Reports of the Signal Service. 



2 Full and interesting accounts of the adaptations of the framework to the 

 external conditions of plants are to be found in the works of Schwendener and 

 Haberlandt. 



3 Westermaier : Monatsber. der k. Akad. d. Wissenschaften Berlin, 1881. 



4 "All are tall and upright columns, but they differ from each other more 

 than do the columns of Gothic, Greek, and Egyptian temples. Some are 

 almost cylindrical, rising up out of the ground as if their bases were concealed 

 by accumulations of the soil ; others get much thicker near the ground like 

 our spreading oaks ; others again, and these are very characteristic, send out 

 towards the base flat and wing-like projections. These projections are thin 

 slabs radiating from the main trunk, from which they stand out like the but- 

 tresses of a Gothic cathedral. They rise to various heights on the tree, from 

 five or six to twenty or thirty feet ; they often divide as they approach the 

 ground, and sometimes twist and curve along the surface for a considerable 

 distance, forming elevated and greatly compressed roots. These buttresses are 

 sometimes so large that the spaces between them if roofed over would form 

 huts capable of containing several persons. Their use is evidently to give the 

 tree an extended base, and so assist the subterranean roots in maintaining in 

 an erect position so lofty a column, crowned by a broad and massive head of 

 branches and foliage " (Wallace : Tropical Nature, 1878, p. 30). 



