. Elements of botany. Botany; Botany. 56 ELEMENTS OE BOTANY. 77. Sieve-Tubes. â Grouped together with the bast fibres of the stem there occur a peculiar and very important set of vessels called sieve-tubes. The student caunot 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 Fi
. Elements of botany. Botany; Botany. 56 ELEMENTS OE BOTANY. 77. Sieve-Tubes. â Grouped together with the bast fibres of the stem there occur a peculiar and very important set of vessels called sieve-tubes. The student caunot 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. 44â Sometimes the walls of sieve-tubes are more or less fully covered with perforations, as shown in Fig. 45. Continuity of the Living Cell- Contents. â It was formerly supposed that cells of plants were entirely shut off from each other while living.' Eecently, careful investigations have shown that very generally, especially in the expanded bases of the leaf- stalks of leaves which move ol their own accord and in siev© cells, there is a direct connec- tion of the contents of one eel] with another. The, protoplasin, 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(* protoplasm of other cells. 78. Longitudinal Section of the Stem. â â 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-ofE ends, appearing as circular or. Fig. 46. â Side View of Part of one of the Medullary Rays of Maple Wood. (Miicli magnified.). Please note that these images are extracted from scanned page images that may have been
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