LEAVES AND LEAF-FORMING AXES. I35 



growth, and persist as small scales, as in Psilotum and in many small leaf-scales of 

 Phanerogams. 



(6) T/ie Leaves usually groiv more rapidly in length than the shoot ivhich produces 

 them does above their insertion (Figs. 106, 107, 108). If the leaves are formed 

 quickly one after another, they envelope and overarch the end of the shoot, and 

 thus form a Bud, in the centre of which lies the leaf-forming punctiim vegetationis. 

 This bud-formation depends at the same time 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 upwards to the stem. 

 It is only when perfectly developed, by the latest extension of their tissue, that 

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

 position in the bud. If the portions of the stem that lie between the insertions 

 of the leaf undergo at the same time a considerable, and often very great extension, 

 the leaves, when escaping from their position in the bud, become placed at a 

 distance from one another, and a shoot results with extended internodes. In such 

 cases the section of the stem in which the leaf-insertion lies usually undergoes 

 a different development from the intermediate portions ; these zones are then 

 termed the Nodes, the intermediate portions the Internodes or interfoliar portions 

 {e.g. Characeae, Equisetaceae, Grasses). If the stem remains entirely undeveloped 

 between the leaf-insertions, it possesses no proper free upper surface, and is endrely 

 enveloped by leaf-insertions (as in Aspidium 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. 107) ; or they may originate only after 

 further development of the stem-tissue, as in IMosses (Fig. 106) and Equisetaceae, 

 where each segment of the apical cell of the stem arches outwards and forms a 

 rudiment of a leaf, so that the leaf-rudiments follow immediately one after another; 

 and it is only by further differentiation that the lower portions of the segment 

 become developed into the free portions of the surface of the stem, as is clearly 

 shown in Fig. 106. The formation of a bud in the way described above is sus- 

 pended when on the one hand the leaves are added very slowly one after another, 

 and on the other hand -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 a slightly developed leaf near the apex, as in the underground creeping 

 shoots of Pteris aqiiilina {vide Book II, Ferns). 



(7) Every leaf assumes a form different to that of the Stem which produces it, and 

 to that of its lateral Shoots. This is usually so conspicuous that no further description 

 is needed. Nevertheless one point must be mentioned which often causes difficulty 

 to the beginner. It not unfrequently occurs that lateral shoots of certain plants 

 present a great similarity to the foliage-leaves of other plants in form and phy- 

 siological properties, as the flat lateral shoots which bear the flowers in Ruscus, 

 Xylophylla, Miihlenheckia platyclada, &c. ; but the course of development shows 



» I consider in Chara, as in Mosses and universally, that the cortex belongs originally to the 

 stem, and not to the leaf. 



