410 PHYSIOLOGY OF GROWTH. 



respectively of Phaseolus. We examine plants of the same age and 

 in vigorous growth. 



A further reason for the rapid growth of etiolated internodes 

 appears in certain cases to lie in the fact that a greater osmotic 

 pressure is set up in their cells than in the cells of normal organs. 

 Some observations of Wiesner and H. de Vries 2 indicate that etio- 

 lated structures are relatively rich in organic acids, and as these 

 bodies are of great significance in the development of turgor, 

 it should be of interest to investigate the matter more closely. 

 Suitable research material would be found e.g. in the epicotyls of 

 Vicia sativa or Phaseolus. It would only be necessary to com- 

 pare the acidity of organs grown in the dark and of organs grown 

 in the light. (For the method see 130.) 



Other observations, however (see Pfeffer, Handbuch, Bd. 2, p. 

 145; de Vries, Jahrbiicher f. wissensM. Botanik, Bd. 14, p. 561, and 

 Wortmann, Botan. Zeitung, 1889, p. 296), tend to show that the 

 osmotic pressure of the cells of etiolated stem structures is not 

 greater than that of normal ones. The whole question requires 

 thorough investigation. 



The active growth of etiolated stem structures is due, therefore, 

 to great turgor-extension of their cells. This in turn is due to 

 increased osmotic pressure of the cell-contents (?), accompanied 

 by comparatively slight power of resistance on the part of the cell 

 membranes. From what has been said it may be imagined that 

 the individual cells of etiolated stems are of greater length 

 than the corresponding cells of normal internodes. And in fact 

 this is the case. 1 have, for example, determined by means of a 

 stage micrometer the length of the pith cells from the middle 

 of normal and etiolated epicotyls respectively of Phaseolus. The 

 former were about O2 mm. in length (it is always necessary to 

 measure a number of the cells, and take the mean value), the 

 latter two to three times as long. 3 



As to why the leaves of most dicotyledons remain so very small 

 in darkness, only the following need here be noted. In darkness 

 the processes which bring about a vigorous surface growth of the 

 cell membranes do not take place. What these processes are it is 

 not exactly known (see, however, the footnote on p. 408). It is only 

 certain that these processes and with them the surface growth 

 of the leaf-cellscan take place when the plants are struck by the 

 rays of light, even if only transitorily. An experiment of Bata- 

 lin's, which may easily be repeated, clearly shows this. We grow 



