656 



This excessi\e length is explained by the modified tension conditions in the 

 stem members. 



The bark, if loosened from the growing part of the stem, contracts; 

 the isolated pith body, on the other hand, becomes considerably longer. It 

 is evident from this that, in the stem, the pith is really the elongating factor, 

 while the rest of the tissue represents the restraining factor. Only when 

 the stem is still very young can the pith satisfy the impulse for elongation 

 because the surrounding tissues are still thin-walled and very easily 

 stretched. They can, therefore, most easily follow passively the strain 

 which the pith exercises. Gradually, however, the elasticity of the outer 

 tissue is entirely lost and the longer pith is now restrained by the thick- 

 walled bark and wood elements. In the latter developmental stage, shorti}' 

 before the stem member ceases growing, the differences in the tissue are 

 equalized, for now the pith cells grow^ broader rather than longer, as a result 

 of the restraining influence of the bark layers and, in this form, become 

 stable since the porous thickening layers are now formed in the cell wall. 



Therefore, the longer the bark elements remain elastic, so much the 

 longer can the pith follow its impulse to elongate and draw the other tissues 

 out wdth it. 



The etiolating plants often resemble juvenile organs and the condition 

 of etiolation, up to a certain degree, can be designated as a permanent juven- 

 ile form. 



After discussing the morphological changes, we have still to consider 

 some metabolistic processes. First of all we will mention the investiga- 

 tions of E. Schulze and N, Castoro^ on Liipiniis alhus. In etiolated seed- 

 lings, the protein content decreases constantly, while asparagin increases ; 

 tyrosin and leucin decrease. At any rate, seedlings grown in the light 

 retain for a long time a high amount of asparagin but contain very little 

 amino acid. 



Palladin's- experiments make it evident that the decreased current of 

 transpiration in etiolated plants causes a too slight absorption of mineral 

 elements, especially calcium. A lack of calcium salts, however, even in 

 leaves rich in proteins, prevents all further development. 



Wiesner^ has shown by numerous experiments that plants gro\vn in 

 the dark are less resistant to atmospheric influences. He found, for 

 example, that seedlings, grown in the light, are much more resistant to the 

 action of rain and water in any form, than seedlings developed in the dark. 



Observations made by Maige* on Ampelopsis and Glechoma show how 

 the material differences come to expression in growth. Diffused light 



1 Schulze, E., u. Castoro, N., Beitrage zur Kenntnis der Zusammensetzung und 

 des Stoffwechels der Keimpflanzen. Zeitschr. f. phys. Chemie, Vol. XXXVIII. Cit. 

 Botan. Centralbl. 1904. No. 47, p. 540. 



2 Palladin, W., Eiweissgehalt der grunen und etiolierten Blatter. Ber. d. 

 Deutsch. Bot. Ge.s. Vol. IX, p. 194. ■ — ErgTiinen und Wachstum der etiolierten 

 Blatter. Ibid. p. 229. 



3 Wiesner, J., Der Lichtgenuss der Pflanzen. Leipzig 1907, W. Engelmann. 

 p. 260. ' "*. 



4 Maige, Influence de la lumiere, etc. Compt. rend. 1898, p. 420. Cit. Bot. 

 .lahresber. 1898. I, p. 587- 



