* C ,OF 



r </n5ITY 



MOVEMENTS OF GROWTH. 



soaking them we lay the seeds on sand moistened with food solution, 

 in small flower-pots. Each pot is provided with four or five seeds. 

 The cultures are placed, as above, in air free from Carbon dioxide, 

 one of them exposed to the light, the other in darkness. After a 

 few days, when germination is fairly advanced, we remove them, 

 carefully from the sand, and divide them up into their separate 

 organs. Seedlings which have not developed in a perfectly normal 

 manner are rejected. For our purpose we need only proceed to 

 examine the hypocotyl and the cotyledons. We determine the 

 weight of these organs, dry them in small glasses at 100 C., and 

 then again weigh them. With care we obtain the following results. 

 The hypocotyls of the plants grown in the light are absolutely 

 poorer in dry substance than those of the plants grown in the 

 dark. The former contain a smaller percentage of water than the 

 latter. The cotyledons grown in light are absolutely richer in dry 

 substance than those developed in darkness. These latter contain 

 a smaller percentage of water than the former.* 



The problem now is to explain clearly why etiolated internodes 

 are usually considerably longer than normal ones, and why the 

 leaves in absence of light are, for the most part, very backward in 

 development. As regards the abnormal elongation of etiolated 

 internodes, it is first to be pointed out, that the membranes of the 

 elements of their tissues (epidermis, collenchyma, bast, wood) 

 remain in an early stage of development, and do not attain their 

 normal thickness. This can be proved by comparing epicotyls of 

 Phaseolus plants grown in the light and in the dark respectively, 

 as regards the structure of their wood elements. The tissue of 

 etiolated internodes must, consequently, be more extensible than 

 that of normal ones, and each individual cell of an etiolated stem 

 will also be able to grow more rapidly than the corresponding 

 cell of internodes developed under the influence of alternating day 

 and night, since its membranes oppose only a comparatively small 

 resistance to the osmotic pressure. It may here also be pointed 

 out, and this stands in immediate relation to what has been said, 

 that, as Kraus first indicated, the intensity of the tissue tension 

 (longitudinal tension) in etiolated internodes, is considerably less 

 than in normal ones. We can satisfy ourselves of this fact by 

 making comparative experiments in the manner described in 149, 

 on the tissue tension of normally developed and etiolated epicotyls 



* I have already shown elsewhere (Versuchsstationen, Bd. 16, p. 212), that 

 etiolated plants are richer in water than plants grown in the light. 



