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.

The relationship between Young's modulus and longitudinal growth has been studied on growing segments of etiolated sunflower hypocotyls. The modulus was determined by means of the resonance frequency method. IAA in a concentration of 2.8 10−5 M produces a decrease in the modulus with a time lag of 4 minutes, while an increase in growth is observable only after 6 minutes. Addition of IAA results in a stronger decrease in the modulus if the segments are placed in a solution of 0.1 M mannitol rattier Hum in water. Through plasmometric measurements it has been found that the elastic extensibility is insignificant compared with the growth. After the addition of IAA, there occurs a marked elongation both in 0.1 M mannitol and in water, and at the same time a decrease in the elastic extensibility of the segments is observed. In the growing segments an increased modulus causes an in creased elastic extensibility, a matter which is directly contrary to the relationship found in a physical system with an applied external force. An explanation of this discrepancy has been given. With an excess of calcium the modulus increases, while the elongation decreases. Calcium deficiency implies both a decreased modulus and a decreased growth. With the addition of 10−3 M Ca(NO3)2 to segments raised without calcium the modulus increases after only 10 minutes, while an increase in longitudinal elongation is observable after 30 minutes. With the addition of IAA to the calcium deficient segments the modulus decreases to the same extent as in an optimal supply o f calcium. The results are discussed with reference to other investigations about elasticity and growth. It has been concluded that plastic extensibility cannot he of great importance in longitudinal growth. Time studies of the auxin effect and I he interaction between auxin and calcium have confirmed the hypothesis that one of the primary actions of auxin consists in a loosening of the cell wall matrix. Calcium always increases Youngs modulus and gives the cell wall a more rigid structure. Furthermore, calcium in a certain concentration is necessary for longitudinal growth.

The effect of auxin and acid solution on Young's modulus (E) and longitudinal growth has been studied on growing segments of Pisum sativum. The modulus has been determined by means of the resonance frequency method. 2.8·10−5M IAA produces a decrease in E within 4 minutes and an increase in length in about 7 minutes. A 10−4M HCl‐solution produces an almost immediate decrease in E and increase in length. The response to the acid solution only persists for about 10 minutes whereas the auxin response continues for hours. Therefore, it is improbable that the hydrogen ions activate a normal enzymatic process leading to increased elasticity and growth. Further the water permeability in potato parenchyma has been investigated by means of the resonance frequency method. Auxin as well as hydrogen ions increase the water permeability, auxin within 2 minutes and hydrogen ions immediately. The lag of the increase in permeability in the auxin solution and the immediate response in the acid solution indicate that auxin and hydrogen ions do not act in the same way, even if an increased membrane permeability is the most probable explanation in both cases. It is suggested that the hydrogen ions act directly on the membrane by changing the membrane potential and thus the permeability, whereas auxin may increase the proton concentration in the membrane enzymatically and thus change the permeability.

By means of the resonance frequency method Young's, modulus has been determined after coumarin treatment of growing segments of etiolated sunflower hypocotyl segments and in maize roots. Coumarin caused a decrease in Young's modulus in both shoot and root tissue. The response was very rapid; in sunflower hypocotyls the decrease in elastic modulus appeared 3 min after application of coumarin. The effects produced by coumarin were similar to those found by auxin. Coumarin increased the rate of water efflux out of potato parenchyma by about 20%. The increase in water permeability was evident within 3 min.