742 GENERAL CONDITIONS OF PLANT-LIFE. 



window during the same time day and night produced nearly twenty grammes of 

 dry substance \ It is a necessary conclusion from the increase in weight of these 

 plants, that in the diffused daylight of the window of a room carbon dioxide is 

 decomposed by the cells which contain chlorophyll, and that this does not take 

 place with great activity. The same conclusion is drawn from the observation that 

 Valltsneria spiralis and Elodea canadensis give off bubbles of gas when the light falls 

 on them for only a rather short time from the northern sky on a clear day, although 

 the exhalation is much more rapid in direct sunlight. In the case of most plants 

 which grow in full daylight, especially our cultivated plants, the increase of weight 

 by assimilation is greatly diminished when they are grown in a window. Within 

 a room itself they usually become exhausted by their own growth in consequence 

 of the defective assimilation, which is not sufficient to replace the material con- 

 sumed in growth and in respiration ; and the plant ultimately dies. Many Mosses 

 on the other hand, and wood-plants of various kinds which grow in the deep shade 

 (as the Wood- Sorrel), are killed by constant exposure to broad daylight; but 

 whether in these cases it is the intensity of the light or the transpiration that is too 

 great, and which of the two is the direct cause of injury, is unknown. Stems which ' 

 attain an enormous length in complete darkness remain perceptibly shorter in the 

 shade of a room ; in a window their growth is still less, and least of all in the open 

 .air in full daylight. The reverse is the case with the leaves of Dicotyledons and 

 Ferns ; in the dark they are often very small ; in deep shade they are considerably 

 larger, and still more so in a light window ; in this position they even appear in 

 many plants {Phaseolus, Begonia, &c.) to attain their maximum of superficial de- 

 velopment, remaining smaller in the open air^. 



(3) Penetration of the rays of light into the plant. In order to determine the 

 dependence on light of certain phenomena of vegetation, it is of special interest to 

 know the depth to which rays of a given refrangibility can penetrate any tissue 

 of a plant, and the intensity with which the different elements of daylight act on 

 particular internal layers. With the exception of the underground parts of plants, 

 stems enveloped in bark, young organs enclosed in leaf-buds, and the like, which 

 are in complete darkness, the assimilating and growing organs are penetrated by 

 light. The deeper the light penetrates, the more does it lose in intensity by ab- 

 sorption, reflexion, and dispersion. This loss however affects the different elements 

 of white light in very different degrees, as was shown by my investigations made 

 in 1859^, at present the only ones on this subject. The rays of greatest re- 

 frangibility are in general almost entirely absorbed by the superficial layers of 

 tissue, while the red light penetrates most deeply. Of successive layers of an 



^ Sachs, Exp.-Phys. p. 21. It must however be observed that the shorter the duration of the 

 light in these cases, the longer was the time of their exposure to the dark in which they again lost a 

 portion of the assimilated substance by respiration. 



"^ The statement made by Famintzin (Mel. biol. vol. VI. p. 73, 1866) that the motile Algae, 

 Chlatnydomonas pulvisculus, Euglena viridis, and Oscillatoria insignis turn both from direct sunlight 

 and deep shade to a light of medium intensity, is contradicted by Schmidt (quoted infra), who found 

 that they always turn to light of greater intensity, and even to direct sunlight. The method' of 

 observation of both authors was however very imperfect. [See also p. 752.] 



' Sachs, Ueber die Durchleuchtung der Pflanzentheile ; Sitzungsber, der Wien. Akad. i860, 

 vol. 43; and Handb. der Exp.-Phys. p. 6. 



