S3 



growth requiring energy, which plants gain by their 

 respiratory process, we find that the healing of wounds 

 is marked in plants as in animals by a local rise in tem- 

 perature. The wound tissue which is developed at first 

 is soft and termed callus; it is that excrescence of cells 

 which is produced as we have seen around the base of a 

 shoot in making cuttings. Within this callus layer imper- 

 vious cork is then formed and this is usually sufficient for 

 the healing of herbaceous plants or soft tissues. In trees 

 and shrubs, however, if the stem or branches are deeply 

 cut, the wound will subsequently be covered up by woody 

 layers as well. The formation of these liquified tissues 

 commences at the margin of the wound and they gradu- 

 ally cover over the entire wounded area. It is thus that 

 branches broken at their base become covered up in nature, 

 and are found as knots buried in the wood. In cultivation, 

 of course, much larger branches are often amputated than 

 usually break off in nature, and though their stumps will 

 ultimately be covered up, it takes a considerable time in 

 the case of a thick branch, and before the wound is closed 

 up there is plenty of time for the wood exposed by the 

 cut to commence to decay. To prevent this from taking 

 place, it is advisable to coat the surface of the stump 

 immediately after the removal of the branch, with tar or 

 some antiseptic substance, which will prevent the entrance 

 of bacteria or of other harmful fungi. 



Injury by lightning, if it is deep and considerable, may 

 be beyond the power of the plant to repair; but where 

 only the outer layers have been damaged, in which case 

 a single longitudinal fissure is generally found running 

 vertically down the side of the stem, callus and cork 

 usually heal up the gap completely. 



Frost is in all probability one of the most frequent of 

 external factors causing injury to plants. Though we 

 cannot as a rule prevent the havoc wrought by frost, it is 

 not without interest to note what is the effect of freezing 

 upon plant structures. It is particularly the young grow- 

 ing parts of plants which are nipped by frost, while the 

 mature leaves are often undamaged. Oil the other hand, 

 somewhat fleshy plants like the Nasturtium, do not resist 

 frost very well. The cell sap containing a good many 

 organic acids and often sugar in solution, does not freeze 

 readily. The denser the sap is, the less danger there is 

 of its being frozen. As a protection against the effect 



