74^ 
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 
Vallisneria 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 tiss-ue 
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^, 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 w^as the time of their exposure to the dark in which they again lost a 
portion of the assimilated substance by respiration. 
2 The statement made by Famintzin (Mel. biol. vol. VI. p. 73, 1866) that the motile Algee, 
Chlamydoinonas pzilvisculus, Euglena viridis, and O&cillatoria 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. 
