424 STRUCTURE OF LEAVES AND CUTICLE. 



be equitant. Now by tracing the middle layer of colourless Cells, 

 d, d, down to that lower portion of the leaf where its two halves 

 diverge from one another, we find that it there becomes continuous 

 with the Cuticle, to the cells of which (Fig. 226, a) these bear a 

 strong resemblance in every respect save the greater proportion 

 of their breadth to their length. — Another interesting variety in 

 Leaf- structure is presented by the Water-Lily and other Plants 

 whose leaves float on the surface ; for here the usual arrangement 

 is entirely reversed, the closely-set layers of green Leaf-Cells being 

 found in contact with the lower surface, whilst all the upper part 

 of the leaf is occupied by a loose spongy Parenchyma, containing a 

 very large number of Air-spaces that give buoyancy to the leaf ; 

 and these spaces communicate with the external air through the 

 numerous Stomata, which, contrary to the general rule (§ 313), 

 are here found in the upper Cuticle alone. 



315. The examination of the foregoing structures is attended 

 with very little difficulty. Many Cuticles may be torn -off, by the 

 exercise of a little dexterity, from the surfaces of the Leaves they 

 invest, without any preparation : this is especially the case with 

 Monocotyledonous plants, the Veins of whose leaves run parallel, 

 and with such Dicotyledons as have very little woody structure in 

 their leaves ; in those, on the other hand, whose Leaves are 

 furnished with reticulated Veins to which the Cuticle adheres (as 

 is the case in by far the larger proportion), this can only be detached 

 by first macerating the leaf for a few days in water ; and if their 

 texture should be particularly firm, the addition of a few drops of 

 nitric acid to the water will render their cuticles more easily 

 separable. Cuticles may be advantageously mounted in weak spirit, 

 or in Grlycerine-Jelly, if it be desired to preserve them. — Very good 

 sections of most Leaves may be made by a sharp Knife, handled 

 by a careful manipulator ; but it is generally preferable to use 

 Valentin's Knife (§ 136) or the Section-Instrument (§ 137) ; taking 

 care in the former case to cut-down upon a piece of fine Cork ; 

 and in the latter not to crush the leaf between the two pieces of 

 cork that hold it, very soft cork being used whenever the delicacy 

 of the Leaf renders this desirable. In order to study the structure 

 of Leaves with the fulness that is needed for scientific research, 

 numerous sections should be made in different directions ; and 

 slices taken parallel to the surfaces, at different distances from 

 them, should also be examined. There is no known medium in 

 which such Sections can be preserved altogether without change ; 

 but some one of the methods formerly described (§ 163) will 

 generally be found to answer sufficiently well. 



316. Structure of Flowers. — Many small Flowers are, when 

 looked - at entire with a low magnifying power, very striking 

 Microscopic objects ; and the interest of the young in such obser- 

 vations can scarcely be better excited, than by directing their 

 attention to the new view they thus acquire of the 'Composite' 



