394 STRUCTURE OF PHANEROGAMIC PLANTS. 



dinall_y, so that what may be considered its under surface is 

 entirely concealed. The two halves are adherent together at 

 their upper part ; but at their lower they are commonly separated 

 by a new leaf, which comes up between them ; and it is from 

 this arrangement, which resembles the position of the legs of a 

 man on horseback, that the leaves of the Iris tribe are said to be 

 equitant. Now by tracing the middle layer of colorless 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 continu- 

 ous with the cuticle, to the cells of which (Fig. 185, a) these 

 bear a strong resemblance, in eveiy respect save the greater pro- 

 portion 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 (§ 248), are here found in the upper cuticle 

 alone. 



250. 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 fui'nished 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 the texture of the cuticle should be particularly 

 firm, the addition of a few drops of nitric acid to the water will 

 render them more easily separable. If it be desired to preserve 

 them, they may be advantageously mounted in weak spirit. 

 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 (§ 106) or the section instrument (§ 107); 

 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, and to use very soft cork 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 difierent direc- 

 tions ; and slices taken parallel to the surfaces, at different dis- 

 tances from them, should also be examined. There is no 

 known liquid, in which such sections can be preserved alto- 

 gether without change ; but water with a small dash of spirit, 



