53 
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 iisstie 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 Sermeete 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. On the other hand, 
somewhat fleshy plants like the Nasturtium, do not resist 
frost very weli. 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 
