_Adventitious buds_ are such as are newly formed from callus or other tissues in places not normally provided with buds, as is often seen on occluding wounds--_e.g._ stool shoots. They may also be developed on roots, a fact utilised in propagating _Bouvardias_, Horse-radish, etc., by means of root-cuttings, and the _suckers_ of Plums and other fruit trees are shoots springing from adventitious buds on roots. Adventitious buds are also common on leaves (_e.g._ _Bryophyllum_, Ferns, etc.), and are frequently induced on them by wounds--_e.g._ _Gesneria_, _Gloxinia_, etc. Even cut cotyledons may develop them, and pieces of leafless inflorescence (Hyacinth), hypocotyl (_Anagallis_), and in fact practically any wounded tissue with a store of reserve materials may be made to develop them: thus they have been found arising from the pith of Sea-kale, and are commonly developed from the cut bulb scales of Hyacinths. _Apospory_ and _Apogamy_ are particular cases of the production of vegetative buds on the leaves in place of sporangia in Ferns (Apospory), and on prothallia in place of Archegonia (Apogamy), in the latter case induced by dry conditions and strong illumination. NOTES TO CHAPTER XXVIII. In addition to the literature quoted in the notes to Chapter XXVII., the student should consult the works on Forest Botany for the scattered information regarding adventitious buds. A good account may be found in Büsgen, _Bau und Leben unserer Waldbäume_, Jena, 1897. For Apospory and Apogamy, see Lang "On Apogamy and the Development of Sporangia upon Fern Prothalli," _Phil. Trans._, vol. 190, 1898, p. 187, where the literature is collected. CHAPTER XXIX. GRAFTS. _Grafting--Comparison with cuttings--Effects of environment-- Relations between scion and stock--Variation in grafts-- Grafting and parasitism--Infection--Pollination--Grafts-hybrids --Predisposition of Natural grafts--Root-fusions._ Grafting is a process which consists in bringing the cambium of a shoot of one plant into direct union with that of another, and is practised in various ways, the commonest of which is as follows: One plant--the _stock_--rooted in the ground, is cut off a short distance above the surface of the soil, and a shoot from the second plant--the _scion_--cut off obliquely with a sharp knife, is inserted into a cleft in the stock, so that the two cambiums (and sometimes the cortex and pith of each as well) are in close contact: the scion is then tied in position, the wounds covered with grafting wax, and the whole left until union of the tissues is completed. This union depends on the formation of _callus_ at the cut surfaces, and the intimate union of the ingrowing cells from each callus. The development of the callus follows the course described for wounds, cuttings, etc., and the union is exactly comparable to the union of the two lips of a healing callus over a wound (see p. 197).