152 EXTERNAL CONFORMATION OF PLANTS. 



are exogenous in their origin, and those on the leaf-stalks of the first-named species 

 arise at an early period, while the leaves are still very young, out of single superficial 

 cells 1. These shoots take root while they still remain in connexion with the mother- 

 leaf, but sooner or later become detached (in Aspidium FUix-mas and Pteris aquilina 

 often only after some years, when they have already acquired considerable strength, and 

 the base of the mother-leaf finally dies off and decays). 



In Phanerogams buds generated on the leaves also occur, although much more rarely. 

 The best known are those which are formed abundantly in the indentations of the 

 leaves oiBryophyllum calycinum ; according to Hofmeister''^ they arise before the complete 

 unfolding of the leaf as small masses of primary parenchyma in the deepest parts of the 

 incisions of the leaf. In the aquatic Utricularia ^vulgaris weak shoots arise, according 

 to Pringsheim^, mostly in the neighbourhood of the axils of the divisions of the leaf; in 

 both cases these shoots are of exogenous origin. Nothing is known of the develop- 

 ment of the buds produced on the leaves of Atherurus ternatus or Hyacinthus Pouzolsii 

 (Doll, Flora von Baden, p. 348). 



(c) Adventitious shoots springing from Roots are ahvays endogenous ; they arise in 

 the neighbourhood of the fibro-vascular bundles or in the cambium, as in Ophioglossum, 

 Epipactis m'lcrophylla, Linaria 'vulgaris, Cirsium arvejise, Populus tremula, and Pyrus Ma/us 

 (according to Hofmeister). 



(d) Adventitious Buds arise moreover in an endogenous manner under peculiar cir- 

 cumstances from older detached leaves or pieces of stem and root, especially when kept 

 damp and in darkness. On this depends the propagation of many plants in gardens, as of 

 Begonias from leaves, Marattias from their thick stipules, &c. Adventitious buds also 

 sometimes appear in considerable quantity in old stems of woody plants, in the cushion 

 which projects between the bark and the wood, especially if the stem is cut off above 

 the root. The branchlets which break out in old stems of Dicotyledons and Mono- 

 cotyledons are, however, often not true adventitious shoots, but old dormant ' eyes ' 

 which have been left behind, having been formed at an earlier period as normal exoge- 

 nous axillary buds, when the stem itself was still in the bud-condition ; they had become 

 enveloped by the bark as the stem increased in thickness, and carried on a feeble ex- 

 istence, until placed in a condition for active growth by a favourable accident, as the 

 removal of the stem above them (Hartig). 



(e) In the genus Isoetes the leaf-forming shoot arises exclusively from the fertilised 

 germ-cell or embryo, and forms neither normal lateral buds out of the stem nor any 

 from the leaves or roots, nor any kind of adventitious buds. 



(f) ^he Normal Formation of Lateral Shoots from the primary meristem of the 

 punctum 'vegetationis of the mother-shoot is endogenous only in Equisetacesc, elsewhere 

 it is always exogenous. The Equisetacea: stand in this respect quite alone in the vegetable 

 kingdom ; with the exception of the weak primary shoot which is developed out of the 

 embryo, all their lateral shoots are of endogenous origin (Fig. 120, KK')] they are 

 developed out of a cell in the interior of the tissue of the stem near to the punctum uege- 

 tationis somewhat later than the youngest leaf-cushions, and afterwards break through 

 the base of the older leaf-sheaths. 



With this exception all normal lateral shoots produced at the vegetative cone of the 

 bud or in its neighbourhood (in the bud) are, like the leaves, exogenous'. 



(g) The lateral shoots which normally arise below the growing apex of a mcther- 

 shoot are always arranged acropetally, like the leaves, with which they exhibit various 

 relationships as to position, age, and number. 



^ Hofmeister, Beitriige zur Kenntniss der gef. Kryptogamen. II. Leipzig 1S57. 

 ^ Hofmeister, Allgemeine Morphologic, p. 42.1. 



3 Pringsheim, Zur Morph. der Utriculaiien ; in Monatsb. der k. Akad. der Wissen. Berlin 1869. 

 * [Leitgeb has recently described the endogenous formation of branches amongst the Hepaticce : 

 Ueber endogene wSprossbildung bei Lebermoosen. Bot. Zeitg. 1872. — Ed.] 



