192 EXTERNAL CONFORMATION OF Pi A NTS. 



leaves, must be mentioned Stipules, Ligular structures, and hood -like out- 

 growths. 



Stipules may be considered as lateral branches of the leaves which arise at 

 their very point of insertion ; they stand in pairs right and left of the base of the 

 principal leaf, either entirely isolated from it or united to it in growth ; each single 

 stipule is usually bilaterally unsymmetrical, and its shape is therefore such that it 

 appears as the reflected image of the stipule belonging to the other side of the leaf. 

 Stipules are not formed until after the origin of the principal leaf, but then grow 

 much more rapidly, and attain their final development at an earlier period ; hence 

 they play an important part in the position of the parts in the bud. In vernation they 

 either extend by their inner margins (those facing the median plane of the leaf) over 

 the back of the principal leaf and cover it outside either partially or entirely, or 

 they extend in front of and over the principal leaf (on the side facing the stem) 

 right and left, and thus cover the parts of the bud next youngest in age. In one or 

 the other of these modes chambers are not unfrequently formed by the stiptiles, 

 in which the formation of the leaves is completed, and which they leave as they 

 expand and unfold ; and the stipules then either also unfold and remain or die and 

 drop oif. 



The term Ligule is applied to a membranous outgrowth on the inner side of 

 the leaf of Grasses at the point where the flat lamina bends out at an angle from 

 the sheath ; it stands transversely to the median plane of the leaf. Similar out- 

 growths are also found elsewhere, as on the petals of Lychnis and Narcissus (where 

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

 in the general term of Ligular structures. In contrast to this outgrowths some- 

 times occur from the posterior (outer) side of leaves, as, for instance, the large 

 hood-like appendages of the stamens in Asclepiadese. 



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

 of one layer of cells. Usually, especially in all large leaves, the tissue is formed 

 of several layers, and then, 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, also called the median vein; and in leaves of 

 more complicated 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 similar 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 w^hich traverse the mid-rib and its stronger branches are enclosed 

 in a thick parenchymatous layer of tissue, the cells of which difi"er from those of 

 the mesophyll. Usually these veins project on the under side of the leaf in the 

 form of cushions, 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, running through the 

 mesophyll of the lamina itself. The kind of venation varies in diff"erent classes 



