664 GENERAL CONDITIONS OF PLANT-LIFE. 



consuming the reserve-material, although in the shade of the room they all pro- 

 duced green leaves. Four other plants of the same species which germinated at 

 the same time grew for three months, exposed for only seven hours each day to 

 the diffused light of a west window in the forenoon ; they weighed when dry 

 nearly 5 grammes. Four other plants which were exposed in a WTSt window from 

 I p.m. till the following morning, and therefore to the afternoon sunshine, weighed 

 also only 5 grammes ; while four other plants which stood in the window during 

 the same time day and night produced nearly 20 grammes each 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 ; although this does not take place with 

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

 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. IMany 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 

 Mhich 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 con- 

 siderably 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 super- 

 ficial development, remaining smaller in the open air^. 



(3) Penetratio7i 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 



^ 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 Algse, 

 Chlamydomonas pulvisciihis., Eitglena viri-dis, 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. Tlie method of observation 

 of both authors was however very imperfect. 



