198 LECTURE XII. 



darkness, though with some inconsiderable abnormalities ; that, however, the organs 

 which are coloured green in the normal condition remain yellow or even colourless in 

 the dark, whilst flowers, fruits and seeds are developed in the normal manner. The 

 most important point here, however, is that such vigorous growth in the dark is only 

 possible when the etiolated shoot is nourished by normal green leaves exposed to 

 the sunlight. If these latter are shut off from the light, or are even much shaded, 

 by which means their assimilation is destroyed or injured, the growth of the parts 

 in the dark box also ceases, or becomes in a high degree abnormal. Stated 

 generally, the phenomenon described follows from the fact that the nutrition of the 

 plant is a function of the chlorophyll in the green leaves ; and that this chlorophyll 

 is only formed under the influence of light, and is enabled by means of the latter 

 to decompose the carbon dioxide of the air, and thereby to produce organic 

 vegetable substance, which becomes transferred from the assimilating leaves into 

 the buds, there to produce new organs even in the dark cavity. From the same 

 fact is to be explained why the culture of plants in dwelling-rooms in general yields 

 such unsatisfactory results. Even when provided with the best soil they remain small 

 and inconspicuous, the older leaves fall off prematurely, and flowers, or it may 

 be fruits, appear only in small numbers or not at all, because the influence of the 

 light through the window on the green nutritive organs is too feeble to bring about 

 a vigorous nutrition. These evils of chamber-culture only come forward the more 

 actively the warmer the plants are maintained ; since the higher temperature forces 

 them to vigorous growth, for which, however, in the feeble light, the corresponding 

 production of constructive materials is wanting. The plants here grow themselves to 

 death, so to speak. It may be allowed here also to interpolate the rule, based on 

 physiology, for the practical culture of plants in green-houses, that so far as plants 

 are concerned warmth chiefly signifies growth, while light, on the other hand, 

 brings about nutrition. Much light with a low temperature usually produces a 

 superfluity of nutritive matters, which the plant bears without injury ; a high tempera- 

 ture with feeble illumination brings about growth without the corresponding nutrition, 

 and is highly injurious to the plant, and in extreme cases even fatal. The 

 too short continuance of daylight also accounts for the fact that in hot, tropical 

 countries many of our native cultivated plants do not flourish ; because, although 

 the high temperature stimulates them, it is true, to vigorous growth, the short 

 tropical days do not allow of a sufficiently energetic nutrition. The larger the green 

 surfaces, especially of the leaves, the greater is the number of rays of light by which 

 •the plant is stimulated to active nutrition. Careful observations have shown that a 

 square metre of the green leaf-surface of a vigorously growing plant produces, 

 in ten sunny hours, 3-8 grams of dry plant-substance by the decomposition of carbon 

 dioxide^. If one reflects how many square metres of leaf-surface a large tree 

 possesses, and that the nutritive activity in one period of vegetation continues with 

 us for about 150 days, it is clear that a tree forms many kilograms of organic sub- 

 stance in the course of a single summer, affording the material for the development 



^ The investigation 'Über specifische Assitnilationscnergie'' ('Arbeiten des bot. Inst, in Wzbg., 

 B. II. p. 346) made by Karl Weber in my laboratory, may be consulted on this subject. See also 

 Sachs, 'Ein Beitrag zur Kenntniss der Ernährungsthätigkeit der Blätter'' in ' Arb. des Bot. Inst.' 

 1884, B. III. p. I, where it is shown that very much more than this may be produccd. 



