Several instances are also known where grafted plants show more or less resistance to such factors of the environment as low temperatures; grafted or budded Roses often suffer much from Erysipheae, and so forth. Much research is still needed to determine how far these matters depend on real alterations in the nature of the graft, or _are only true for the localities in which the experiments have been made_, a point which has, I think, been overlooked by all observers. Grafted plants are apparently very much exposed to injury by slugs, insects, and the invasions of parasites during the healing of the callus and the fusion process. Here again it must not be overlooked that the callus is, so to speak, a tit-bit of luscious, thin-walled, succulent tissue; and, like all wounds, the graft affords entrance to parasites such as _Nectria_ and Ascomycetes of various kinds, under circumstances very favourable to their invasion. _Natural Grafts._--It is by no means an uncommon event to find the branches of Beeches, Limes, and other trees which have been accidentally brought into contact during growth, joined where they cross. As they press one against the other, they become naturally grafted, by that form of the process known as _inarching_: except that in artificial inarching the operator cuts off the cortical tissues of the two branches and brings their cambial surfaces together, whereas in nature the cambiums only come into contact after the destruction by pressure, or slight abrasion, of the entrapped intervening tissues. The fusion occurs, in fact, exactly as in the burying-in of a nail or wire, referred to on p. 211. Natural grafts are very common among the roots of trees, and possibly explain some queer cases of the apparent revivification of stumps of trees not usually given to forming abundant stool shoots. It is regarded as probable in some old forests that the majority of the roots of trees of the same species are linked up together by such natural grafts, a probability not diminished by the fact that such roots cross at many points, and are easily grafted. NOTES TO CHAPTER XXIX. The student should read Bailey, _The Nursery Book_, 1896, for details regarding the practice of grafting, and facts in abundance can be obtained from the pages of the _Gardeners' Chronicle_. Concerning graft-hybrids and the variations of grafted plants see Jouin, _Can Hybrids be obtained by Grafting?_ and especially Daniel, "La Variation dans la Greffe," in _Ann. des Sc. Naturelles_, S. VIII., Vol. 8, 1898, p. 1, and the literature there collected. The whole subject is largely controversial, and much work remains to be done. CHAPTER XXX. LIFE AND DEATH.