285 



Growth in length and thickness of the petiole of richardia. By Kath- 

 ERiNE E. Golden. 



The subjects of growth and tension have such a close inter-relation that 

 it is difficult to separate them in order to determine how much of the ex- 

 pansion taking place in a plant organ is due to growth and how much to 

 tension. It is well to understand first what is meant by growth and what 

 by tension. 



Growth is a permanent change that takes place in an organ, usually ac- 

 companied by a change in bulk, and is dependent on constructive and 

 destructive metabolism, an adequate supply of moisture, and a tempera- 

 ture between certain definite degrees. Tension, on the other hand, is a 

 state of turgidity of the cells, /. e., the cells are capable of absorbing water 

 in such quantities that it causes considerable stretching of the cell walls ; 

 this will cause a pressure from within on the cell-wall, and where a cell 

 forms part of a tissue, there will also be a pressure from without from the 

 surrounding cells, thus setting up tensions in the various tissues. These 

 tensions will, of course, cause the dimensions of an organ to change. 



From work done by Kraus" it has been determined that organs dimin- 

 ish in bulk (estimated on diameters) from morning until afternoon, and 

 increase until towards dawn. This depends upon the quantity of water 

 which the organ contains, and again on gain by absorption and loss by 

 transpiration. 



To determine the laws governing some of these changes I made a set of 

 experiments upon the petiole of Richardia. The petiole is made up of 

 thin-walled parenchyma, having large, regular, inter-cellular canals and 

 small, fibro-vascular bundles scattered through it, and enclosed by some- 

 what smaller parenchyma cells containing chlorophyll, alternating with 

 groups of collenchyma cells, the whole being surrounded by the epidermis 

 which has very thick outer walls. It can be seen from its structure that it 

 would necessarily contain much water and air, and respond very readily 

 to tensions. 



To show how much water the plant may contain a set of experiments 

 made by Ungert who replaced the root pressure by a column of mercury 

 in one case gave 2(3.5 grms. of water from 6 leaves in 11 days, and in an- 

 other 36 grms. from 4 leaves in 10 days. 



In my work I used self-recording auxanometers to make measurements, 



-Kraus. Quoted in Vines Physiology of Plants, p. 405. 

 tUnger. Quoted in Physiology of Plants, Vines, p. 92. 



