369 



LEAF. 



LEAF. 



370 



a flat leaf is connected at its base with the simultaneously-developed 

 wing or angle of the axis, so that the full-grown leaf appears to be 

 directly continuous with this. Such a leaf ia said to run down the 

 axis (folium decurrens), as in Carduus, or, by a wholly unfounded 

 fiction, a leaf blended by growth with the axis (axis folio adnatus). 

 Where several leaves arise simultaneously, or almost simultaneously, 

 at about the same height upon the axis, the bases of the leaves 

 become gradually approximated during development ; and here it 

 may readily happen that they approach so close that the same process 

 occurs between the bases of two different leaves, as has been already 

 described in the two borders of one and the same leaf. Thus it 

 happens, that leaves, which in their origin and at their summits 

 are free and isolated, in their ulterior development and at their bases 

 form an undivided whole (leaves grown together, ' folia connata '). 

 The leaves of Lonicera Caprifolium, afford an example simplest 

 and easiest to trace. Two foliaceous organs which originate one above 

 the other on the same axis (as the petal and stamen), or a leaf and 

 the bud developed in its axil (as the bract with the flower-stalk in the 

 Lime) may grow together one above the other in the same way. 



A process almost diametrically opposite to this may occur, where a 

 leaf ia developed, but becomes suddenly arrested in its development 

 in a way as yet unknown, whether through mere mechanical pressure 

 or flomf other cause, by the more rapid and powerful development of 

 the contiguous leaves ; so that either the little original papilla escapes 

 notice, ou account of its relatively minute size in the full-grown part, 

 or the little prominence actually becomes effaced by the subsequent 

 development of the part, or, finally, the little rudiment of a leaf dies 

 and gradually decays. In this case the leaf is said to be abortive : 

 an instance easily traced is afforded by the third perigonal leaf of 

 Carcjc, which aborts in this way, while the two others form the 

 so-called Utriculus ; and not only may whole leaves become abortive 

 in this way, but even individual portions of a leaf of which the rudi- 

 ments already exist : thus it is not at all rare for the so-called stipules 

 to become disproportionately developed in the rudimentary leaf, 

 whUe the proper leaf, restrained in its growth, gradually disappears 

 from sight. The Bud-Scales (Ramenta) on the perennial buds of 

 Corylut atellana may serve as examples, being in fact nothing else 

 than the stipules of an abortive leaf. 



The same influence to which the parts closely crowded in the bud 

 are subject, may merely cause the unsymmetrical development of the 

 two halves of a particular foliar organ, so that one side, or that part of 

 the leaf lying on one side of the midrib, assumes a form different 

 from that of the other half, of which the species of Begonia afford a 

 striking example. 



The nascent leaf consists, like all nascent parts of vegetables, of 

 cellular tissue ; determinate cords of cellular tissue are first gradually 

 organised into vascular bundles ; and in fact this process proceeds 

 from the vascular bundles of the axis, and advances gradually into the 

 leaf. In many foliar organs, especially the parts of the flower, no 

 vascular bundles are ever formed. The vascular bundles of the leaves 

 are distinguished by the most inconveniently chosen expressions, 

 Nerves or Veins (Nervi, Vena;). Ribs (Coste) is a more correct 

 expression. In Monocotyledons with undeveloped internodes, the 

 whole of the vascular bundles together ( ? ) of the internode, bounded 

 above by the leaf, pass into the leaf. In all other plants, many at 

 least of the vascular bundles entering the leaf are minor twigs of the 

 vascular bundles of the axis ; in the Dicotyledons proceeding exclu- 

 sively, in great part, from the borders of the loop of the vascular 

 bundles of the axis. The course of the vascular bundles in the leaf 

 depends essentially on the form of the latter. In flat leaves, petioles, 

 or vaginal portions, the vascular bundles lie in one plane ; in relatively 

 thick leaves, 4c., they lie scattered (Palms) or in a circle (species of 

 Aloe, Maembryanthemum). The vascular bundles rarely run separately 

 through the whole leaf (as in the last-named) : they mostly anastomose 

 in various ways with each other by lateral branches ; frequently in the 

 petiole, in such a manner that all the vascular bundles entering it 

 unite into a single one, and then separate again iu the blade of the 

 leaf. The form of the combinations is very varied : in many Mono- 

 cotyledons the branches are short, going off at right angles ; in others, 

 and in most Dicotyledons, more varied, so tliat a net with polygonal 

 meshes is formed. 



The vascular bundles of the leaves are progressive bundles, and 

 they are so formed that (regarding the leaf as passing off horizontally 

 from the axis) the oldest parts lie above, the youngest below. In the 

 lower part also a cambium layer exists in the Dicotyledons : in the 

 lower part liber-bundles accompany the vascular bundles, and in the 

 under part the vascular bundles, in relatively thin and flat leaves, 

 project above the surface (probably in consequence of gradual develop- 

 ment), while the upper part of the leaf appears level. 



The parenchyma of the leaf is developed in the most varied manner; 

 in general, in thick solid leaves, it is composed externally of small 

 crowded cells containing more chlorophyll, internally, of larger and 

 looser cells filled with aqueous juices. Very often the outer layer 

 passes into a tissue, the cells of which are elongated ill a directiou 

 vertical to the surface of the leaf, are applied closely, almost without 

 trace of intercellular passages, and thus are pretty sharply distinguished 

 from the rest of the parenchyma, and occur in the whole of the peri- 

 phery of the leaf, not only in round and triangular leaves, but also in flat 



HAT. HUT. DIV. VOL. III. 



ones, as in many Australian Myrtacece. In flat leaves, especially of Dico- 

 tyledons, there is very often a separation into two layers, the upper of 

 which has the cells elongated perpendicularly to the surface, as just 

 mentioned, filled with much chlorophyll, while the lower is composed 

 of looser, globular, or, still more frequently, spongiform parenchyma 

 containing little chlorophyll. In thick coriaceous or fleshy leaves, for 

 instance iu species of Picus and Peperomia, one or more layers of cells 

 containing little but watery juices, often lie between the upper layer 

 and the epidermis ; more rarely, iu like manner, at the under surface 

 of the leaf. 



Besides these there appear at given places, or dispersed in the 

 parenchyma, according to special peculiarities of the plant, spiral 

 fibrous cells, very thick, and closely porous cells, and cells containing 

 peculiar juices and crystals. Milk-vessels and passages, receptacles 

 for gum, oil, aud resin, are also found, and isolated liber-bundles, the 

 last especially in the thin elongated leaves of Monocotyledons. Air- 

 canals and air-cavities are found in the leaves ; the last very regularly 

 and beautifully arranged. 



All foliar organs, soon after their origin, exhibit a delicate epithe- 

 lium, which, in plants vegetating under earth or under water, is 

 converted in time into epiblema, and in those vegetating above the 

 surface is converted into epidermis. Some parts of flowers are clothed 

 with a peculiar sort of covering, holding an intermediate station 

 between epithelium and epidermis. [FLOWER.] To the epiblema 

 stomata are wanting. The epidermis is commonly provided with them. 

 [STOMATES.] In flat horizontal leaves they are very frequently wanting 

 on the epidermis of the upper side, and they are usually only found 

 where a thin or spongiform cellular tissue is present beneath the 

 epidermis ; in floating leaves, on the contrary, the upper epidermis 

 only has stomata, and through the upper layer of condensed elongated 

 parenchyma air-canals pass into the under and thinner layer of paren- 

 chyma ; as occurs also in leaves that are surrounded with dense 

 elongated cellular tissue. All parts usually known as appendages to 

 the epidermis are also found occasionally on the leaves : even the cork 

 structure is sometimes found on the petioles of long-enduring leaves, 

 as, for example, in some species of Pathos and Ficua, as well as on the 

 leaves of Crastula, Bryophyllum, &c. 



The cells of the epidermis are usually filled with a clear watery 

 fluid, which on the under surface of the leaf is sometimes coloured 

 (red). They more rarely present crystals, and yet more seldom offer 

 any peculiar matter, as resin, or the like. The form of the epidermis- 

 cells is determined by the form of the leaf; long slender leaves 

 usually present their epidermis-cells elongated in the same direction. 

 The lateral walls of the epidermis-cells are often curved in the form 

 of waves, but this peculiarity has been too little investigated to be 

 explained at present. 



The following is a complete view of the foliar organs : The floral 

 parts of a plant are distinguished from the other foliar organs, and 

 are termed Flower-Leaves (Phylla), whilst other leaves are termed 

 True Leaves (Folia seusu stricto). 



1. True Leaves (Folia). 



A. Seed-Leaves (Cotyledons), generally round or flat, fleshy, little 

 divided, and never compound. 



B. Stalk, or Stem- Leaves (Folia Caulina). Their forms are very various, 

 as has been shown in the foregoing paragraphs. Those immediately 

 following the Cotyledons are usually simple ; the next more perfect ; 

 and again, as they rise into the viciuity of the blossoms, they become 

 again more simple. Filiform leaves, or parts of leaves, when they 

 twine around foreign objects, are termed Tendrils (Cirrhi), as in Pisum, 

 Clematis, &.c. ; those which are stiff and pointed are termed Spines 

 (Spinae) ; very concave leaves that exhibit the form of a cup or pitcher 

 are termed Pouches (Asci), as in Nepenthes, Sarracenia, Utricidaria, &c. 

 According to their various positions they are again distinguished from 

 the True Leaves generally : 



a. Leaves of the Inflorescence (Folia Floralia). Indistinguishable 

 from the stem-leaves, but bearing in their axils a blossom or a simple 

 inflorescence. 



4. Bracts (Bractea;). Leaves different from the stem-leaves, and 

 bearing in their axils a blossom or a simple inflorescence ; for instance, 

 the scarlet-red leaves of the Salvia Jforminum,. To these belong tho 

 Glumae of Grasses, which are simply two bracts (which have commonly 

 no blossoms in their axils), and the leaves which surround the capitula 

 of the Composite. A number of bracts, inclosing an inflorescence, 

 are also termed an Involucre (Involucrum). The quickly-drying 

 bracteae of the Composites are termed Scales, or Chaff (Paletc), a word 

 altogether superfluous. 



c. Bracteoles (Bracteoltc), distinct from the stem leaves, and standing 

 beneath the blossom, but upon its axis; for example, the two leaves 

 under the blossom of the Aconitnm, &c. 



C. Bud-Scalos (Tegmenta). The very simple, mostly membranous, 

 and quickly-falling outer leaves of a bud which remains for a length 

 of time uuexpauded. (See hereafter, undsr the Bud.) 



2. Flower-Leaves (Phylla). 



A. Perigonial Leaves (Phylla Perigonii). 



2 u 



