CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. 



213 



they form the so-called Corona), on the leaves of Allium, &c., and may be included 

 in the general term of ligular structures. Outgrowths sometimes occur from the 

 posterior (outer) side of leaves, as, for instance, the large hood-like appendages 

 of the stamens in Asclepiadeae. 



It is only in some Muscineae that the tissue of the leaf consists throughout of 

 'single layer of cells. Usually, especially in large leaves, the tissue is composed 

 of several layers, and, in vascular plants, is distinguished into epidermis, paren- 

 chymatous fundamental tissue, and fibro-vascular bundles. The fundamental tissue 

 is termed mesophyll ; the system of the fibro-vascular bundles running into the 

 leaf forms the so-called venation. In the leaves of many Mosses which otherwise 

 consist of only one layer, there runs in the middle from the base towards the 

 apex a bundle of several layers, called the mid-rib ; in leaves of more com- 

 plicated structure there is also usually a mid-rib which runs from the base to the 

 apex of the lamina, and divides it more or less symmetrically into two halves. 

 The same occurs in every lateral leaflet or in every branch or lobe of the lamina ; 

 from the mid-rib spring the lateral veins which run to the margin of the leaf. In 

 larger leaves, especially those of Dicotyledons, the fibro-vascular bundles which 

 traverse the mid-rib and its stronger branches are enclosed in a thick paren- 

 chymatous layer of tissue, the cells of which differ from those of the mesophyll. 

 Usually these veins project on the under side of the leaf, and the larger the 

 whole lamina the more strongly are they constructed (especially the mid-rib). 

 The finer veins, on the contrary, consist of single fibro-vascular bundles, often 

 branching extensively, which run through the mesophyll of the lamina itself. 

 The kind of venation varies in different classes of vascular plants, and is often 

 very characteristic of large groups. This will be explained more in detail in the 

 proper place. 



In Characeae, Muscineae, and Vascular Cryptogams, all the leaves of a plant 

 are usually similar, being either simple or segmented in the same manner, although 

 the segmentation, especially in Ferns and Rhizocarps, is simpler in young than in 

 the large leaves of mature plants. But it also happens, even in Cryptogams, that 

 leaves of very different forms are found on the same plant. Thus some Muscineae 

 form colourless minute leaves on the underground creeping shoots, and in the neigh- 

 bourhood of the organs of reproduction they often produce leaves of a different 

 shape from those on the rest of the upright parts. In the same manner among 

 Ferns the leaves on the underground shoots (stolons) of Struthiopteris germanica 

 are represented by thin membranous scales, which are replaced on the upright end 

 of the stolon by large green pinnate leaves. In Salvinia, among Rhizocarps, each 

 "'whorl consists of two simple roundish leaves which rise into the air, and one that 

 hangs down into the water and consists of filiform branches. Even in Coniferae and 

 ' Cycadeae variation in the leaves of the individual plant is much more common ; 

 while in Monocotyledons and Dicotyledons the shapes of leaves become extra- 

 ordinarily variable, not only on the same plant but often on the same axis. 



The two most common forms of leaves are the scale- or 'cataphyllary' leaves * 

 and the foliage-leaves. 



* [Henfrey, in his translation of Braun's ' Rejuvenescence in Nature' (Ray Soc, Botanical and 

 Physiological Memoirs, 1853), first proposed to render the terms Hochblatt, Nicdcrblall, and 



