. Botany of the living plant. Botany. 142 BOTANY OF THE LIVING PLANT very slight organic framework containing water. The mechanical effectiveness of the internal turgor of the cells, and the insufficiency of mere partitioning of a young or succulent part is shown by com- paring a crisp fresh leaf with one which has withered, or has been plasmolysed. There is, however, another factor which increases the mechanical effectiveness of succulent parts in the young state, viz. the mutual tensions of tissues. If a fresh young stem of Sunflower or Elder, or any extending part of an herbaceous plant, be
. Botany of the living plant. Botany. 142 BOTANY OF THE LIVING PLANT very slight organic framework containing water. The mechanical effectiveness of the internal turgor of the cells, and the insufficiency of mere partitioning of a young or succulent part is shown by com- paring a crisp fresh leaf with one which has withered, or has been plasmolysed. There is, however, another factor which increases the mechanical effectiveness of succulent parts in the young state, viz. the mutual tensions of tissues. If a fresh young stem of Sunflower or Elder, or any extending part of an herbaceous plant, be slit longitudinally into quarters, these take strong curves. The outer surface of each quarter becomes concave, the inner faces of each quarter convex. The curves become more marked after the cut stem has been steeped in water. These curves show that the relations of the tissues in the living stem are not passive. (Fig. loi, 2a, 2b.) That the phenomenon is one of turgor of the cells may be shown by allowing the slit stem to wither, or by plasmolysing it with a salt solution, when the curves dis- appear, and the parts become limp. On the other hand, if the several tissues be completely separated from a measured length of a fresh stem, and be themselves measured after separation, the column of pith will be found to have elongated, and the outer tissues to have contracted. To bring them back to their original state the pith would have to be compressed and the outer tissues stretched. (Fig. loi, a.) This is in fact their condition in the growing stem. The pith tends to elongate, but is held back by the outer and firmer tissues, which are thus kept tense. The relation of the inner and outer tissues is then analogous to that of the v,-all and protoplast in the turgescent cell, and the mechanical effect is t he same. It is thus seen that the firmness of succulent stems is due in large degree not only to the turgor of the individual cells, but also to the mutual tension of its tissues
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
