. The Botanical Class-Book and Flora of Pennsylvania. iate spaces, or mesh-es, of this frame work; and a third, a thin, cuticular expan-sion, inclosing the other two, forming the epidermis, orcovering for both surfaces of the leaf. PHYSIOLOGICAL STRUCTURE OF Li:A\ IS. 4U 136. The frame work or vascular portion of the leaf(Fig. 24,) is composed of vascular tissue and spiral v<which fulfil the same office in the leaf as in the stem, notonly giving firmness and support to the delicate cellular ap-paratus, but also serving for the conveyance and distributionof the sap. These veins divide and Su
. The Botanical Class-Book and Flora of Pennsylvania. iate spaces, or mesh-es, of this frame work; and a third, a thin, cuticular expan-sion, inclosing the other two, forming the epidermis, orcovering for both surfaces of the leaf. PHYSIOLOGICAL STRUCTURE OF Li:A\ IS. 4U 136. The frame work or vascular portion of the leaf(Fig. 24,) is composed of vascular tissue and spiral v<which fulfil the same office in the leaf as in the stem, notonly giving firmness and support to the delicate cellular ap-paratus, but also serving for the conveyance and distributionof the sap. These veins divide and Subdivide far beyond thelimits of unassisted vision, until the threads of woodyare reduced to separate fibres ramified throughout thepulp, so as to supply every portion with the sap they contain,which, having reached the utmost limits of the edge of theleaf, double back upon themselves, pervade the lower surface,and are again collected into the petiole, through-which theyare finally returned into the baric, constituting the vessels ofthe latex. 73. 137. The cellular PORTION, or parenchyma, exists intwo layers, arranged upon a regular plan, which varies iudifferent parts of the-leaf, according to the different condi-tions in which it is placed. In all those leaves which expandhorizontally, one surface being turned upwards and the otherdownwards, these two layers arc dissimilar in structure; butin those leaves where the lamina is vertical, as iu the Iris,they do not materially differ. The upper stratum is composedof one or more compact layers of oblong cells, placed in buc-Iia position that their long diameter is perpendicular to theupper surface, (Fig. 73, h. I.) while the lower stratum is com-posed of oblong cells arranged longitudinally, and so looselycompacted as to leave numerous vacant spaces between thecells (c. c), which communicate with the air by means of thu^tomato or brmthina pores, (Tier. 74, a. b.).E 50 PHYSIOLOGICAL STRUCTURE OF LEAVES. 138. The epidermis of the
Size: 2303px × 1085px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthornol, bookcentury1800, bookdecade1850, booksubjectbotany
