667 



tions, parallels the transpiration. Investigation showed that not only the 

 absolute production of the young plants was essentially more energetic in 

 the light, but that also a square centimeter of leaf surface developed a 

 greater amount of substances. A weakening of the light, by means of col- 

 ored media, through which the rays must pass, acts similarly to the removal 

 of light by placing it in the dark. In yellow light, assimilation and transpi- 

 ration are more energetic than in blue light; at least the majority of experi- 

 ments favor this^. 



The energy of production of plants and also the mode change with the 

 decrease of light and this change is expressed, not only in the metamorphic, 

 but also in the metabolic structure. 



The well-known experiment of covering lea,ves in the light with a 

 stencil pattern, which leaves free some rather larger surface figures, remov- 

 ing the green from these leaves after some days by means of alcohol, and 

 then wetting them with iodine solution, is a simple illustration of the action 

 of light. All parts of the leaf, which have been exposed to the light, look 

 blue because of the action on the starch which had been formed in the light. 

 This experiment is of interest inasmuch as it shows how locally limited the 

 action of light is. Only the part which had been illuminated formed starch 

 and no starch passed over into the darkened, adjacent part. The most 

 important thing, according to this, is that the green parts of the plant must 

 themselves work over their constructiv^e materials if they should continue 

 to live. 



It has been mentioned already that the mobilized reserve substances 

 pass into the young, entirely darkened shoots a certain distance from the 

 tubers and seeds. If the distance is too great, however, the shoots finally 

 die from starvation. They breathe up more respiratory material than they 

 receive in the form of sugar, etc. Some of Miiller-Thurgau's^ experiments 

 show, for example, that the starch, when dissolved, passes over into sugar, 

 which is used up partly for construction and partly in respiration. Grape 

 leaves, which contain 2 per cent, sugar and as much starch, were cut off and 

 their petioles put in water. The container was set in a room at zero de- 

 grees. Nine days later all trace of the starch had disappeared. Since the 

 respiration of the grapevine, however, at zero degrees is very slight, the 

 sugar, produced by the solution of the starch m the dark, cannot have been 

 used up. in respiration and must, accordingly, have accumulated in the leaf. 

 As a fact, investigation shows 4 per cent, sugar in the leaves. 



Thus, placing in the dark wall promote the formation of sugar in the 

 organs as against the formation of starch. If, as is frequently the case in 

 growing plants out of doors, an actual temperature decrease takes place 

 with the decrease in light, it means a blocking of sugar in the assimilator}' 

 tissues. 



1 Compare Hellriegel, Beitrage. p. 378. Nobbe, Versuchsstationen XXVI, p. 354. 

 Flahault, Bot. Centralbl. 1880, p. 932. Deherain, Bot. Zeit. 1873, p. 494,^ 



2 Miiller-Thurgau, tJber den Einfluss dei' Belaubung- auf das Reifen der Trau- 

 ben. Weinbaukongress zu Durkheim a. d. H. 1882. 



