CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. 213 
they form the so-called Coro7ta), 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 Muscinese that the tissue of the leaf consists throughout of 
a 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 inesophyll ; 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 rmis 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 veifis which run to the margin of the leaf. In 
larger leave's, 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 Characese, Muscinese, 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 Muscinese 
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 germattica 
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 dov/n into the water and consists of filiform branches. Even in Conifer^e and 
Cycadese 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. 
^ [Henft-ey; in his translation of Braun's ' Rejuvenescence in Nature' (Ray Soc, Botanical and 
Physiological Memoirs, 1853), first proposed to render the terms Hochblatt, Niederblatt, and 
