Summary Calcium as a plant nutrient is characterized by its relatively high content in the plant coupled with a requirement not much higher than that of a micro nutrient element and an exceedingly uneven occurrence in soils. The difficulties in defining its actions are accentuated by a weak biochemical activity. In ecological conditions the secondary consequences of variations in calcium content may be more striking than the direct ones. Electron‐microscopical studies have revealed that calcium is required for formation and maintenance of lamellary systems in cell organellae, a fact which might suffice to explain its indispensability for meristematic growth. Calcium is required for cell elongation in both shoots and roots; the common experience that it inhibits shoot elongation is certainly due to calcium additions far above actual requirement. It must be assumed for a rational interpretation of cell elongation that the fundamental mechanism is the same in shoots and roots. The one action which can be ascribed with certainty to calcium is a stabilizing of the cell wall with an increase in rigidity, an effect which, with over‐optimal supply, may lead to growth inhibitions. The function is, however, necessary for the normal organization of cell walls. Calcium has, on the contrary, no significant effect on the synthesis of cell wall compounds but appears to act on their proper incorporation into the cell wall. The growth‐active calcium may be bound not only to pectins but also to proteins and nucleoproteids in or in close contact with the cell wall. The supposition that calcium interacts directly with auxin in the cell wall has not been verified and does not seem very probable. There are reasons to believe that the points of action of calcium and auxin in the cell wall differ, auxin inducing growth by wall loosening and calcium establishing new wall parts. For submerged organs it may be necessary to consider an indirect effect of calcium on growth by its regulation of cytoplasmic permeability and thus affecting the exudation of growth‐active compounds. The ecological problem is to characterize calcifuges (acid soil plants) from calcicoles (base soil or calcareous soil plants). Growth inhibitions on acid soils depend upon poisoning by A13+ and Mn2+. Opinions differ as to what extent this can be antagonized by calcium. Lime‐induced chlorosis in calcifuges depends upon iron deficiency or iron inactivation in the plant. No acceptable explanation is given, but it might be related to an interaction of calcium carbonate, phosphorus, and iron. A hypothesis that it is linked to formation of organic acids is not tenable in the given form. Plants react to the calcium ions in the concentrations found in soils. Calcifuges have a low calcium‐optimum for growth and show growth inhibition at high concentrations. Calcicoles have a high optimum for growth. Calcifuges are resistant to aluminium poisoning. Attempts made to explain the differences in calcium uptake and generally in salt uptake are tentative only, and relevant data are l...

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AbslraelThe relations between longitudinal growth, Young's modulus, luigor. water potential, and tissue tensions have been studied on growing internodes of etiolated pea seedlings in an attempt to apply some physical concept.s to the growth of a wellknown plant material. The modulus has been determined by the resonance frequency method and expressed as En,^,,,,. It increases nearly proportional to the turgor pressure and is at water saturation more than 50 times higher than at plasmolysis. Etisaue 's higher in the epidermis than in the ground parenchyma. Indoleacetic acid causes a decrease in Etj^suo-Other properties have heen studied on intact and split segments of internodes in solutions of graded mannitol additions. -The following tentative picture of the normal course of the growth has been obtained. Anxin induces growth both in the periphery (epidermis) and in the central core (parenchyma) nnder a decrease in Etiss,,^. This is followed hy tin increase of Etj^s^R which is independent of auxin hut depending upon the turgor pressure. It is assumed to involve internal structural changes of the cell walls of the type of creep. The rapid f,'row(h takes place in a dynamic system with a low water jrotential despite favourable water eonditions. Epideimis and parenchyma grow equally rapid without tissue lensions. -Such can be produced artificially by splitting of .segments and water uptake. The parenchyma thereby loses its sensitivity to auxin. This is the background of the split stem test for auxin. -Etigg,,^ increases when growth is slowing down, probably owing to both synthesis of wall substance and structural changes wilhin the wall. The cells attain a more static condition with E,js^,,,> higher in epidermis than in parenchyma. This leads to the normal tissue tensions. -The result ngrees with growth according to the multi-net-principle. The cause of the low water potential and low tnrgor is discussed with reference to th& dynamic nature of both growth and water transport and a probably low matric potential of tbe streaming water. 'Ihc decrease in En^sue following auxin addition is small but is the net difference between an auxin-induced decrease and an increase through the assumed creep.

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