TREES AND SHRUBS. 385 



trunk or branch, so that no portion of the amputated branch may 

 remain. 



An exposed stump or stub left when a branch breaks off cannot 

 possibly heal over of itself, and if it project far from the trunk or 

 branch, the cambium or inner bark cannot produce enough callus to 

 cover and protect it, so that decay of necessity sets in, and the stump 

 dies back, possibly to communicate disease to the trunk ; even if it 

 does heal up at the base, a bad knot is formed in the timber. A clean- 

 cut, smooth and hard surface, such as is produced by a sharp pruning- 

 saw, is much preferable to a rough and jagged one, since it affords less 

 lodgment for water or germs, avoids crushing or tearing, of the cam- 

 bium, and assists the progress of the protecting and healing callus. 



In the case of a large wound, cork and new cambium are produced 

 by the callus at the edges of the wound, which does not close up until 

 cushion after cushion of callus has grown over it, and this may take 

 several years. Though the growth of the callus cannot be hastened by 

 "dressing" the wound, the latter can be protected from moisture and 

 fungus-spores by applying tar or lead-paint. 



The old wood exposed oy the wound, which generally becomes dark 

 in colour, does not grow with the new wood formed by the callus, 

 hence a deep cut e.g. an inscription made in the wood of a tree can 

 be found years afterwards, covered by annual layers of new wood. 

 Any hard body, e.g. a nail or the stem of another tree, may be enclosed 

 in a tree by causing the cortex to split and a callus-layer to be formed 

 which grows over the body and covers it with new wood. 



Wounds made in herbaceous stems, roots, tubers, leaves, fruits, etc. , 

 are healed in a much simpler way. In general, whenever the inner 

 tissues are laid bare by injury a layer of cork is formed by the exposed 

 cells which are living and capable of growth ; the wounds covered in 

 this way are easily recognised by the brownish patches of cork. A 

 potato- tuber, for instance, is covered by a layer of cork, and a similar 

 layer is soon formed on a piece cut out of the tuber and kept under 

 a bell- jar or tumbler ; try this simple experiment, and if you have a 

 microscope cut sections to see the cork-layer in (1) an uninjured tuber, 

 (2) a piece cut out and allowed to heal. 



Familiar operations which depend upon callus formation are 

 pruning, grafting, budding, and propagation by cuttings. 



377. Scots Fine or " Scotch Fir " (Pinus sylvestris) is 

 usually easy to distinguish from other Pines by the bluish- 

 green foliage-leaves (" needles ") arranged in pairs ; the bark, 

 orange-coloured except at base of trunk, where it is dark ; the 

 pointed cone, with a dull surface. 



The tree rarely exceeds a hundred feet in height. In 

 plantations or forests it shows a tall branchless stem (any 

 signs of branches formerly present ?) with an umbrella-like 

 crown, but in the open it keeps the lower branches for many 

 years. The bark on the upper part of the trunk peels off in 

 s. B. 25 



