HEDGE AND DITCH 155 



greater than that of a solid rod of the same weight per 

 unit of length. 1 I suppose that it does not matter much at 

 the moment whether the central space is filled by pith or 

 by a simple air-space, as in the stalks of cow-parsnep or 

 the haulms of grasses. A continuous cavity would however 

 be dangerous to a stem which was destined to thicken 

 and last for many years ; it would give great opportunities 

 to wood- eating insects and wood- destroying fungi ; if the 

 branch should ever be broken across, water would get in 

 and do great mischief. Pith is much safer, and it costs 

 the plant next to nothing, for it contains hardly any solid 

 matter. Here again we find animals using the same ex- 

 pedients as plants. A porcupine- quill or the shaft of a 

 feather is very like an elder-shoot packed with pith. In 

 all of them great firmness is attained with the smallest 

 possible weight. 



As the elder- trunk increases in diameter the pith remains 

 unchanged ; does this mean that the disposition of the 

 wood becomes less and less advantageous as the stem gets 

 bigger ? Not necessarily, but the great relative rigidity 

 ceases to be so important a consideration when the ascend- 

 ing stem has gained two or three inches of thickness. 

 Henceforth it will have rigidity enough for every purpose ; 

 the property of chief importance to the old stem is its 

 weight-supporting power. This is mainly a question of 

 the number of the wood-fibres, and hardly at all of their 

 arrangement. The hollow cylinder placed vertically can 

 support no greater load than a solid rod of the same weight 

 per unit of length. The strength of the branches which 

 stand out more or less horizontally, is of less vital import- 

 ance to the tree ; they will grow as far as they can safely 

 grow, and that must suffice. It is by no means an 

 unqualified advantage to cover as great an area as 

 possible, and the elder very likely thrives as well in its 

 modest way as if it had the far-spreading boughs of a 

 beech-tree. 



1 Round the Year, pp. 154, 262. 



