382 THE MICROSCOPE AND ITS REVELATIONS. 



leaves float on the surface; for here the usual arrangement is entirely re- 

 versed, 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 ( 382), are here found in the upper epiderm alone. 1 



384. The examination of the foregoing structures is attended with 

 very little difficulty. Many epiderms 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 Monocotyledons generally, 

 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 epi- 

 derm 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. 

 Epiderms may be advantageously mounted either in weak spirit, or in 

 glycerine-jelly. Very good sections of most leaves may be made by a 

 sharp knife, handled by a careful manipulator; but it is generally prefer- 

 able to use the Microtome, placing the leaf between two pieces either of 

 very soft cork or of elder-pith or carrot, or imbedding it in paraffine ( 189). 

 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 ( 206) will generally be- 

 found to answer sufficiently well. 



385. Flowers. Many small flowers, when looked-at entire with a low 

 magnifying power, are very striking Microscopic objects; and the interest 

 of the young in such observations can scarcely be better excited, than by 

 directing their attention to the new view they thus acquire of the ' com- 

 posite ' nature of the humble down-trodden Daisy, or to the beauty of 

 the minute blossoms of many of those Umbelliferous Plants which are 

 commonly regarded only as rank weeds. The Scientific Microscopist, 

 however, looks more to the organization of the separate parts of the flower;, 

 and among these he finds abundant sources of gratification, not merely 

 to his love of knowledge, but also to his taste for the beautiful. The 

 general structure of the sepals and petals, which constitute the 'perianth* 

 or floral envelope, closely corresponds with that of leaves; the chief dif- 

 ference lying in the peculiar change of hue which the chlorophyll almost 

 invariably undergoes in the latter class of organs, and very frequently in 

 the former also. There are some petals, however, whose cells exhibit 

 very interesting peculiarities, either of form or marking, in addition to 

 their distinctive coloration; 2 such are those of the Geranium (Pelargo- 



1 See the classical Memoir by Ad. Brongniart on the Structure of Leaves, in 

 "Ann. des Sci. Nat.," Tom. xxi. (1830) pp. 420-458. 



2 See especially Mr. Tuffen West ' On some Conditions of the Cell- Wall in the 

 Petals of flowers,' in " Quart. Journ. of Microsc. Science," Vol. vii. (1859), p. 22. 



