1^6 MORPHOLOGY OF MEMBERS. 



bundles of the stem, that although they are actually continuous, their independent 

 origin is still easily seen in radial longitudinal sections through the apices of young 

 stems. In both cases the development of the first vessels usually commences in 

 the region where the. bundle that bends out into the leaf unites with that of the 

 stem. In addition to the Common Bundles formed in this way, fibro-vascular bundles 

 may also arise in the stem, as has already been described in Sect. i8, which belong 

 to it alone, and are therefore termed Cauline Bufidles. Cauline bundles may be 

 formed at an early period of the growth of the stem, as in Piperaceae, Nycta- 

 gineae, &c., or only as a consequence of increase in thickness, when the leaves have 

 already long been developed, or have even fallen off, as in Aloineae, Menispermaceae, 

 &c. (see Sect. i8). 



(6) The Leaves usually grow more rapidly in length than the parent shoot above 

 their insertion (Figs. ii6, 117, 118). If the leaves are formed rapidly one after 

 another, they envelope the end of the shoot, and thus form a Bud, in the centre 

 of which lies the leaf-forming growing point. This production of a bud depends 

 also on the more rapid growth of the outer or under side of the leaves in their 

 young state, by which they become concave on the inner (afterwards the upper) 

 side, and adpressed to the stem. It is only by the extension of their tissue that 

 the leaves ultimately turn outwards in the order of their age, and thus escape 

 from the bud. If the portion of the stem between the insertions of the leaves 

 undergoes at the same time a considerable extension, the leaves then become 

 placed at a distance from one another, and a shoot results with elongated inter- 

 nodes. In such cases the section of the stem in which the leaf-insertion lies 

 usually developes in a different manner from the intermediate portions ; these zones 

 are termed the Nodes, the intermediate portions the Internodes, as in Characese, Equi- 

 setaceae, and Grasses. If the stem remains entirely undeveloped between the nodes, 

 it possesses no proper exposed surface, but is entirely covered by the leaf-insertions, 

 as in Nephrodium Filix-mas ; but more commonly this is only apparently so from 

 the internodes being very short, as in many palm-stems. The internodes may be 

 present immediately after the first formation of the leaves, when the consecutive 

 leaves or leaf-whorls appear at considerable distances in height from one another, 

 as in Chara ^ and Zea (Fig. 117); or they may originate only after further develop- 

 ment of the stem-tissue, as in Mosses (Fig. 116) and Equisetaceae, where every 

 segment of the apical cell of the stem forms a rudiment of a leaf, so that the 

 leaf-rudiments follow immediately one after another; and it is only by further 

 cell-formation, growth, and differentiation that the lower portions of the segment 

 become developed into the exposed portions of the surface of the stem, as is 

 clearly shown in Fig. 116. The formation of a bud in the way described above 

 does not take place when on the one hand the leaves are developed very slowly 

 one after another, or on the other hand when the stem grows rapidly in length 

 between the youngest leaf-rudiments or even before the appearance of the youngest, 

 so that there is always only one slightly developed leaf near the apex, as in the 

 underground creeping shoots of Pteris aquilina (see Book II, Ferns). 



^ I consider in Chara, as in Muscinese, and in fact universally, that the cortex belongs originally 

 to the stem, and not to the leaf. 



