288 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



light upon plant growth and structure, many different kinds of reactions have 

 been found to take part, such as oxidation, polymerization, decomposition, and 

 even synthesis— the last in the presence of hydrocyanic acid, which is widely 

 distributed in plants. 1 These processes are very rapid in the presence of inor- 

 ganic salts. 2 They have not yet been studied in plants excepting in con- 

 nection with the activity of chlorophyll, but there is no doubt that they must 

 be important. Neuberg was right when he wrote: "These rapid chemical re- 

 actions caused by light may furnish a clue to the chemical processes that under- 

 lie phototropic responses, and even to the chemical nature of sunlight effects, 

 in general upon both plants and animals" (Neuberg, cited just above). 



It is well known that the seeds of certain plants germinate only in darkness, 3 

 while seeds of other plants, and certain spores, germinate only in light. In the 

 latter case, as in growth phenomena generally, light acts not only as a stimulus 

 that releases a reaction but also supplies energy that is necessary for the process 

 in question. This statement seems to elucidate the fact, among others, that 

 the light requirement of many seeds depends upon internal conditions such as 

 the stage of maturity of the seeds; light is especially requisite for the germina- 

 tion of seeds that have not been allowed to reach complete maturity. Many 

 spores that ordinarily show a low percentage of germination in darkness germi- 

 nate very well when iron salts of organic acids are supplied. 4 Finally, Wiesner's 

 observations [see note I, page 276] on the optimal light conditions (Lichtgenuss) 

 for various plants have shown that the light requirement increases as the tem- 

 perature of the surroundings falls. The various characteristic forms and struc- 

 tures resulting from etiolation are thus to be regarded as correlations between 

 the different parts and organs of the plant, these being due partly to a deficiency 

 in organic assimilation products, partly to a cessation or those photo-chemical 

 processes that are independent of chlorophyll, and partly to a modified distribu- 

 tion, in the plant body, of water and dissolved mineral substances, which results 

 from reduced transpiration. All these conditions must also influence the com- 

 position of the cell sap, which in turn controls turgor and the properties of the 

 protoplasmic membranes. 



Not only a complete lack but also an inadequate supply of light produces 

 modifications in plant form and structure. If plants of the same species are 

 grown, some in bright sunlight and some in diffuse light, the two groups exhibit 

 very different structures, this difference being especially pronounced in the 

 leaves. 5 Leaves grown in diffuse light are always thinner than those grown in 



1 Ciamician, G., La chimica organica negli organismi. 99 p. Bologna, 1908. Idem, 1908. [See note 

 2, p. 35.] 



2 Neuberg, 1908. [See note 3, p. 35.] 



a Kinzel, Wilhelm, Ueber den Einfluss des Lichtes auf die Keimung. "Lichtharte" Samen. (Vor- 

 laufigeMitteilung.) Ber. Deutsch. Bot. Ges. 25 : 269-276. 1907. Idem, Die Wirkung des Lichtes auf die 

 Keimung. (Vorlaufige Mitteilung.) Ibid. 26: 105-115. 1908. Idem, Lichtkeimung. Einige besta- 

 tigende und erganzende Bemerkungen zu den vorlaufigen Mitteilungen von 1907 und 1908. Ibid. 26: 631- 

 645. 1908. Idem, Lichtkeimung. Weitere bestatigende und erganzende Bemerkungen zu den vorlaufi- 

 gen Mitteilungen von 1907 und 1908. Ibid. 26: 654-665. 1908. 



4 Laage, A., Bedingungen der Keimung von Farn- und Moossporen. Beih. Bot. Centralbl. 21': 76-115. 



J907. 



6 Dufour, Leon, Influence de la lumiere sur la forme et la structure des feuilles. Ann. sci. nat. Bot. VII, 



5: 311-413- 1887. 



