384 TREES AND SHRUBS. 



carefully studied. In many cases this is more easily done in winter, 

 when deciduous trees show their bare branches. 



Note also the depth of the shade cast by the tree when in full 

 foliage, and the effect this has on the nature and habit of the plants 

 forming the undergrowth ; the mode of branching shown by the tree 

 when growing with others in a clump or in a wood, and that shown by 

 the same kind of tree when growing by itself in an open situation ; 

 the differences in size and in texture of the leaves well exposed to 

 light, and of those growing in shade ; the tendency of the leaves on 

 each twig to form a "leaf -mosaic" (i.e. to vary in size, shape, etc., so 

 as not to shade each other). 



Some further points relating to the biology of trees may be mentioned 

 here. One often sees on high exposed places inland, as well as near 

 the coast, trees which have grown obliquely, sometimes almost 

 horizontally, after reaching a certain height ; in other cases the trees 

 growing in such situations are erect, but growth has occurred almost 

 entirely on one side. If you see examples of such trees, try to discover 

 the causes of their bent or one-sided growth. Note the direction of 

 the bend or that of the well-grown side of the tree, then find out the 

 directions and characters (whether dry or moist, cold or warm) of the 

 prevailing winds. At what time of year will the prevailing wind have 

 most influence on the tree's growth, and in what way ? 



376. How Wounds Heal. You will often notice on the 

 trunks of trees curious ring-like cushions. It is easy to find 

 out the meaning of these cushions and to trace their history, 

 especially if you have seen foresters at work cutting off 

 branches. Under whatever circumstances and for whatever 

 reasons (find out all you can on these points) this " tree 

 surgery " has been practised, the amputation- wound soon 

 begins to heal up, under proper conditions. The cambium 

 (Art. 212), which has been exposed by the cutting, produces 

 a mass of growing tissue, known as callus, which in time 

 rolls over and covers the wounded wood (which cannot heal 

 of itself). If the wound is a small one, the callus-cells soon 

 meet at the centre and form a continuous tissue which pro- 

 duces cork at the surface and (being continuous with the 

 ordinary cambium) wood internally, covering up the stump. 



The callus protects the exposed wood by excluding water, bacteria, 

 and fungi, which set up decay ; all exposed surfaces, whether caused by 

 pruning-knife or saw or by the breaking-off of branches, offer a foot- 

 hold for disease and decay, and once a wood -rot fungus enters it may 

 grow inside the tree after the wound is quite covered. Hence, for 

 the wound to heal properly and as quickly as possible, the limb to be 

 removed should be cut off close to and perfectly even with the parent 



