. Elements of botany. Plants. 56 ELEMENTS OF BOTANY. 77. Sieve-Tubes. — Grouped together with the bast fibres of the stem there occur a peculiar and very important set oi vessels called sieve-tubes. The student cannot easily make these out from sections of ordinary stems, but it* is not diffi- cult to understand their structure in a general way. These tubes arise from the partial union of large cells which stand in rows, united end to end, as shown in Figs. 42, 43. The partitions between adjacent cells gradually become perforated with holes, forming a sieve-plate, like that shown in Fig. 44.,
. Elements of botany. Plants. 56 ELEMENTS OF BOTANY. 77. Sieve-Tubes. — Grouped together with the bast fibres of the stem there occur a peculiar and very important set oi vessels called sieve-tubes. The student cannot easily make these out from sections of ordinary stems, but it* is not diffi- cult to understand their structure in a general way. These tubes arise from the partial union of large cells which stand in rows, united end to end, as shown in Figs. 42, 43. The partitions between adjacent cells gradually become perforated with holes, forming a sieve-plate, like that shown in Fig. 44., Sometimes the walls of sieve-tubes are more or less fully covered with perforations, as shown in Fig. 45. Continuity of the Living Cell- Conteiits. — It was formerly supposed that cells of plants were entirely shut off from each other while living. Recently, careful investigations have shown that very generally, especially in the expanded bases d¥ the leaf- stalks of leaves which move oi liiMliSlli'ir&illftiM] their own accord and in sieve- cells, there is a direct connec- FiG. 46. —Side View of Part of one tiou of the COntentS of OUC CcU of the Medullary Rays of Maple .,, ,, „, , Wood. (Much magnified.) With another. The jprotoidasm, or semi-fluid layer with which all active cells are lined, and in which their life and working- power resides (Chapter XIII), extends in delicate threads through the cell walls, and connects in all directions with th(^ X)rotoplasm of other cells. 78. Longitudinal Section of the Stem. — The knowledge of stem-structure that can be gained from a longitudinal section of any kind of wood depends upon the way in which the sec- tion is cut; that is, whether it is at right angles to the annual rings (^radial section), or parallel to the rings (tangential sec- tion). The wood-cells, of which the student has in the cross- section seen only the cut-off ends, appearing as circular oi. Please note that these images are extracted from scanned pag
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectplants, bookyear1896
