ARRANGEMENT OF MECHANICAL TISSUES 291 



hollow stem, so common in herbaceous plants, is itself an 

 example of the hollow column. We see this in such plants 

 as the Umbelliferas and the grasses. Such a hollow tube has 

 this weakness, that, if bending takes place over a considerable 

 length, it may buckle ; this is largely overcome if the tube 

 is partitioned, and such partitioning, more or less complete, 

 usually occurs at the nodes. In some aquatics diaphragms 

 occur at more frequent intervals (Snow, 1914). In many 

 rushes the hollow is filled with a spongy pith of little strength, 

 but serving as a latticed matrix for the tube ; in Juncus 

 articulatiis and others the pith is confined to numerous 

 partitions. It is interesting to note that buckling of grass 

 stems is infrequent, unless in violent gales, except in the 

 cultivated cereals, lodging of which by heavy rain or wind 

 is a source of much loss. Here we are dealing with artificial 

 races bred by selection for heavy cropping, though, of 

 course, the quality and resistance of the straw is also con- 

 sidered. They are plants in which the balance of load to 

 support has been deliberately altered in favour of the former, 

 and this explains their weakness as compared with natural 

 races. Shading accentuates the weakness. 



The bending strain is especially great in the case of 

 horizontal branches bearing, towards their extremities, a 

 load of leaves, and here we find special modifications. At 

 its junction with the stem the branch springs upwards, and, 

 in older branches, the development of wood is increased 

 on the lower side. The branch is borne on a buttressed 

 arch. The secondary thickening of the branch is asym- 

 metrical. In general the development of wood is stronger 

 on the upper side in broad-leaved trees, and on the lower 

 side in conifers (" red " wood). The elements of the lower 

 wood in the conifers are more strongly thickened. In some 

 broad-leaved trees, such as the oak, the upper wood is dis- 

 tinguished by a greater proportion of mechanical elements ; 

 in others, like the beech, the structure of the wood is the 

 same above and below. In all cases the eccentricity means 

 increased resistance to bending, by withstanding strain 

 better in the broad-leaved trees, and stress in the conifers. 



