Introduction to structural and systematic botany, and vegetable physiology, : being a 5th and revedof the Botanical text-book, illustrated with over thirteen hundred woodcuts . s (presently to be described) ; and when Ave consider the powerfulaction of the sun to promote evaporation, especially in dry air; andthat the thin walls of the cells, like all vegetable membrane, allowof the free escape of the contained moistiu*e by transudation. Thecompactness of the cells of that stratum which is piesented immedi-ately to the sun, and their vertical elongation, so that each shallexpose the least poss
Introduction to structural and systematic botany, and vegetable physiology, : being a 5th and revedof the Botanical text-book, illustrated with over thirteen hundred woodcuts . s (presently to be described) ; and when Ave consider the powerfulaction of the sun to promote evaporation, especially in dry air; andthat the thin walls of the cells, like all vegetable membrane, allowof the free escape of the contained moistiu*e by transudation. Thecompactness of the cells of that stratum which is piesented immedi-ately to the sun, and their vertical elongation, so that each shallexpose the least possible surface, obviously serve to protect theloose parenchyma beneath from the too powerful action of directsunshine. This provision is the more complete in the case of plantswhich retain their foliage through a season of drought in arid re-gions, where the soil is usually so parched during the dry season,that, for a long period, it affords only a scanty supply of moisture tothe roots. Compare, in this respect, a leaf of the Lily (Fig. 221),where the upper stratum contains but a single layer of barely oblongcells, with the firm and more enduring leaf of the Oleander, the. upper stratum of which consists of two layers of long and narrowvertical cells as closely compacted as possible (Fig. 222). So dif- FIG. 221. A magnified section through the thickness of a minute piece of the leaf of theWhite Lily of the gardens, showing also a portion of the under side with some breathing-pores. 148 THE LEAVES. ferent is the organization of the two strata, that a leaf soon perishesif reversed so as to expose the lower surface to direct sunshine. 264. A further and more effectual provision for restraining theperspiration of leaves within due limits is found in the Epidermis,or skin, that invests the leaf, as it does the whole surface of the vege-table (69), and which is so readily detached from the succulent leavesof such plants as the Stonecrop and the Live-for-ever (Sedum) ofthe gardens. The
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Keywords: ., bookauthorgra, bookcentury1800, bookdecade1860, booksubjectbotany
