DICOTYLEDONS. 



5^3 



cannot be placed in the same category as the cases just mentioned of extra-axillary 

 branching, where large leaves in the axils of which buds are also formed exist 

 near the extra-axillary lateral branches. Here, on the contrary, as for instance in 

 Crucifer?e and the capitulum of many Compositae, the formation of leaves on the 

 axis of the inflorescence is itself entirely suppressed ; there are no leaves in the axils 

 of which the branches could stand. The branches are hov/ever produced as if the 

 leaves were actually there ; and there are reasons for supposing that we have here 

 a case of abortion of the bracts in the same sense as the abortion of the posterior 

 stamen of Labiatae (p. 480), Musaceae (Fig. 397, p. 548), &c. Since it is common 

 for the hypsophyllary leaves on the inflorescence to remain very small and to 

 disappear early, it would not be surprising, according to the theory of descent, that 

 functionless organs of this kind should at length entirely disappear, their deve- 

 lopment being in such cases altogether suppressed, while the lateral branches which 

 belong to them (according to the theory of descent typically axillary) should be 

 strongly developed. 



Adventitious buds are rare in Dicotyledons, as they are in Phanerogams 

 generally. Those which are commonly formed with an exogenous origin in the 

 indentations of the margins of the leaves of Bryophyllum calycinmn are v/ell known, 

 and serve to propagate the plant. They sometimes occur (according to Peter- 

 hausen') in Begoiiia coriacea in the form of small bulbs on the peltate surface of 

 the leaf where the i)rincipal veins radiate'-. On the adventitious buds on the leaves 

 of Utricularia, Pringsheim's treatise already quoted may be consulted. Adventitious 

 buds more often spring from roots, e. g. in Anemo7ie japonica, Linaria vnlgaris, 

 Cirsiiim arvcnsc, and Popiiliis ircmula, according to Irmisch^ The shoots which 

 spring from the bark of the older stems of trees must not at once be set down as 

 the development of adventitious buds ; since the numerous dormant buds of woody 

 plants may long remain buried and yet retain their vitality. 



The Leaves of Dicotyledons exhibit a greater variety both in their position and 

 their form than those of all other classes of plants put together. The ordinary 

 phyllotaxis of seedlings begins with a whorl of two cotyledons, and continues 

 either in decussate pairs or passes into a distichous arrangement or into whorls 

 consisting of larger numbers or spiral arrangements with the most various angles 

 of divergence. More simple arrangements, especially that of decussate pairs, are 

 generally constant in whole families, the more complicated arrangements usually less 

 constant. Axillary branches' usually begin with a pair of leaves which are either 

 opposite or alternate, and stand right and left of the median line of the mother-leaf. 



It is quite impossible to give in a short space even a general account of the 

 forms of leaves, even apart from cataphyllary leaves (scales on underground 

 stems and those which envelope persistent buds), hypsophyllary leaves or bracts, 



^ Beitr:;ge zur Entwickelung der Brutknospen (Hameln 1S69). where various examples are also 

 given of axillary buds of Dicotyledons which form deciduous gemmx ; :,s\n Polygonnvi viviparrmi, 

 Saxifraga granulata, Dentaria bulbifera. Ranunculus Ficaria, &c. , r i -r i 



■' [The common method of propagating Begonias is by culling or tearing tne leaf, which, if then 

 placed on moist soil, produces buds on the edges.— Ed.] 



3 [Irmisch, Bot. Gaz. Ill, pp. 146 and 160.] 



002 



