THE GROWTH OF ROOTS AND STEMS. 173 



plant-tissues taking place chiefly through the stomates (in 

 leaves and herbaceous stems) and the lenticels (in woody 

 stems) . 



202. The "Skeleton" of the Plant. We might com- 

 pare the hard parts of the stems, roots, and leaves of a plant 

 with the skeleton of an animal, since these hard parts support 

 the plant and help to keep it firm and strong. 



Why are the hard tissues of the stem arranged in a hollow cylinder, 

 or tube, like the pillars that support the galleries in buildings ? Why 

 are they packed together towards the centre in the case of the root, 

 forming a tough central cord ? Why are the chief bundles in the leaf- 

 stalk arranged like the letter U in a cross-section ? Why are the chief 

 veins (bundles) of the leaflet arranged like a feather, and why do they 

 project on the lower side of the leaflet? Why are the finer leaflet- 

 veins arranged in a network ? 



Most of these questions can be answered by considering the results 

 of simple experiments on the bending of a beam of wood (or other 

 material) firmly fixed at one end and loaded at the other. The lower- 

 ing of the loaded free end is (a) proportional to the load, (&) propor- 

 tional to the cube of the length of the beam, (c) inversely proportional 

 to the breadth of the beam, (d) inversely proportional to the cube of 

 the depth of the beam. 



From (c) and (d) we see at once why it is so much more advan- 

 tageous to increase the depth than to increase the breadth of a beam. 

 For when the beam is bending, the upper side lengthens and the lower 

 side shortens, but the middle portion of the beam neither lengthens 

 nor shortens. Exactly the same applies when the beam is set upright 

 instead of horizontally, and force is applied to it from one side. Hence 

 it is a good plan to put the hard material near the outside, and this is 

 done in the girder, which is I-shaped in cross-section. If we want an 

 upright rod to resist bending before winds coming from north, east, 

 south, and west, we need two joined girders crossing at right angles. 

 In such a structure the central part is clearly under little strain as 

 compared with the outer parts, therefore it would be better to put all 

 the material on the outside and make it into a tube (to resist the bend- 

 ing effects of winds coming from all directions). 



Clearly, then, the strength of the upright structure (stem) is, using 

 the same amount of material, very greatly increased if the hard material 

 is placed towards the outside^ in the form of a tube. If you think over 

 these points in the "mechanics" of plants you will see how the plant 

 successfully meets the mechanical needs of the various parts and at the 

 same time the need for economy in the amount of material used to 

 make the parts strong enough. The leaflets have to be supported and 

 spread out to air and light ; the stem has to carry the leaves and 

 branches and has also to resist undue bending before the wind. 



The root, on the other hand, being embedded in the soil has no 

 weight to support, but it has to keep a firm hold of the soil (the root- 



