36 



BEITISH FLOWEPJNG PLANTS 



and hence they are termed dicotyledonous. Herbert 

 Spencer ingeniously suggested that this explains the 

 general connection between monocotyledonous germina- 

 tion and so-called endogenous growth in the stem, on 

 the one hand ; dicotyledonous germi- 

 nation and exogenous growth, on the 

 other. The fact that it throws light 

 on these fundamental relations adds 

 very greatly to the probability of the 

 hypothesis, which, however, cannot 

 yet be said to be generally accepted.^ 

 Considering the sizes of trees, their 

 height and weight, it is really wonder- 

 ful how they are able to support 

 themselves and resist the force of the 

 wind. The roots fix them firmly in 

 the ground, and the framework of the 

 stem is so constructed as to give 

 Fig. 21.— Frond strength- immense Strength. The hard "bast" 



ened by midrib. ,, n i-i i _ci Tir 



cells, from which woody fibres aiiier 

 but little, are said to have in some trees a bearing 

 capacity equal even to that of steel. Besides the 

 woody fibres, the stem of a tree contains bark, cork, soft 

 tissue or parenchyma, vascular and other bundles, pith, 

 etc. These are by no means arranged 

 at random. The forces acting on and 

 bending the stem of a forest tree tend to 

 extend the tissues on the convex, and 

 compress them on the concave side. In 

 the centre they vanish. The strengthen- 

 ing material must obviously be applied 

 where the force to be resisted is greatest, 

 i.e. near the bark. The arrangement 

 may be compared to that by which a girder is strength- 

 ened at the side with flanges. In other species there is 

 another or there may be several series of woody bundles, 

 each with a group of vascular cells, always lying on the 

 inner, or central, side. 



^ See Herbert Spencer, Principles of Biology, vol. ii. 



Fig. 22. 



