THE LEAF-BUD. 71 



presenting themselves under special conditions. They usually 

 occur on organs in a very active state of vitality, subjected to 

 stimulating external conditions, especially where, through natural 

 or artificial operations, there is an absence or insufficiency of 

 normal buds to carry off the developmental energy of the plant 

 or organ. 



Adventitious buds may be produced from any part of the plant. With 

 regard to those produced on old stems, as in pollarded trees, or those which 

 occur on subterraneous stolons, as in the Rose, Ash, &c., it is not always 

 easy to decide without dissection whether the buds are really adventitious 

 or merely latent axillary buds stimulated into development ; but true ad- 

 ventitious buds do occur. The production of adventitious buds on true 

 roots has been frequently observed, as in Pyrus japonica, Madura auran- 

 tiaca, Paulownia imperialis, &c. ; and the Anemone japonica is commonly 

 propagated by cuttings of the root. The formation of adventitious 

 buds on leaves is a still more remarkable physiological phenomenon. It 

 has been observed chiefly in succulent leaves, but it is not exclusively con- 

 fined to them. When it takes place, the first sign of development is the 

 production of adventitious roots, followed by the formation of a cellular 

 nodule which subsequently assumes the character of a bud. Among 

 natural examples, the leaves of Cardamine pratensis have been observed 

 to form adventitious roots on the lower side when lying upon wet ground, 

 and even to produce buds ; the leaves of several Ferns, such as Woodwar- 

 dia radicans, root at the end, and produce buds which propagate the plant ; 

 and many similar instances might be cited. Artificial production of buds 

 on leaves is now a familiar fact, under the influence of heat and moisture, 

 not only on the scales of bulbs, but on the green leaves or even fragments 

 of the leaves of Bryophyllum, Echeveria, Gloxinia, Gesnera, Hot/a, &c. ; 

 the Orange and the Aucuba japonica may also be propagated by their 

 leaves. Sometimes the leaves produce rootlets alone, and remain stationary 

 without having force enough to develop a bud. 



The formation of adventitious buds on leaves, especially in Bryophyllum, 

 where a number are often produced, arranged on the margin, is of great 

 interest in connexion with the theories of the structure of ovaries and 

 the origin of the ovules. 



Bud-scales. The bud which continues the growth from the 

 plumule of a germinating plant (fig. 122), and the axillary buds 

 produced during a season of active growth, are composed of rudi- 

 mentary leaves ; but the winter- or resting buds formed on most 

 deciduous trees and shrubs of temperate climates present the 

 modified foliar organs called bud-scales (perulce}, analogous to the 

 scales of bulbs and other subterraneous buds of herbaceous plants 

 (figs. 123 and 124). Buds without scales are called naked. The 

 scales, when present, are mostly of leathery or membranous texture, 

 and are often clothed more or less densely with hairs, which are 

 sometimes glandular and produce a resinous or glutinous secretion, 

 which exudes when the buds swell. 



