. Botany of the living plant. Botany. MECHANICAL CONSTRUCTION OF PLANT-BODY 141 them together with lime as in Halimeda. These are to be regarded as con- cessions to the mechanical imperfection of the non-septate construction. Cases of non-septate tissues exist in the body of some of the higher plants, but as they are embedded in other tissues they are not exposed to mechanical demands. Examples are seen in the latex-cells and vessels, as in the Euphorbiaceae and Cichoriaceae; and in the young embryo-sacs of the Flowering Plants. All ordinary plants of large size are septate. This is specially
. Botany of the living plant. Botany. MECHANICAL CONSTRUCTION OF PLANT-BODY 141 them together with lime as in Halimeda. These are to be regarded as con- cessions to the mechanical imperfection of the non-septate construction. Cases of non-septate tissues exist in the body of some of the higher plants, but as they are embedded in other tissues they are not exposed to mechanical demands. Examples are seen in the latex-cells and vessels, as in the Euphorbiaceae and Cichoriaceae; and in the young embryo-sacs of the Flowering Plants. All ordinary plants of large size are septate. This is specially necessary where they live exposed in the air, and are thus subjected to greater strains than if floating in water. In the embryonic region the cells are seen to divide into equal parts, the newer cell-wall. ? ^ ? P X Fig. 100. Diagram illustrating the plan of arrangement of cell-walls in the apex of the stem of an Angiosperm. ^^=axis of construction. ££ = external surface. PP=pri- clinal curves. AA =anticlinal curves. (After Sachs.) being inserted at right angles on the older walls. This leads to a cell-net the exact detail of which depends upon the external form of the part (Fig. 100). The disposition of the walls is from the first such as to give added mechanical strength, but in the young cell the walls are extremely thin, and are composed of pliant material. In the young shoot the mechanical strength is almost entirely dependent on the turgor of the individual cells. When they are tense their walls do not act as mere props or stays, as do the floors or partitions of a house; but the turgor gives individual rigidity to each cell, and through them collectively to the whole part which they construct. A high mechanical effect is thus gained in a succulent structure with extreme economy of material. This may be illustrated by the case of the Lettuce. We have seen that a crisp Lettuce suitable for a salad gives by weight 96 per cent, of water, and only four per cent, of organic
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
