78 



THE INFLUENCE OF CLOUD AND FOG. 



There are some parasitic plants, says Marie-Davy (1881 and 1882), 

 -that require only moisture and warmth in order to vegetate. They 

 mature and propagate while entirely cut off from sunlight, but they 

 derive this power from organic matter or cells that have been pre- 

 viously formed by the action of sunshine upon the plant on which 

 ^-the parasite itself feeds. 



Similarly certain bulbous plants will flower and mature in dark- 

 ness, but in doing so the bulb itself is wholly consumed and dies ; the 

 plant lives on organic matter that was elaborated and stored up by 

 its parent and predecessor in preceding years Avhen it had sunshine 

 to do the work for it. If a new bulb is to be formed as a basis for the 

 flowering of the next year then the present bulb and plant must be 

 allowed the necessary sunlight. 



Similarly the seeds of the annuals sprout and nourish their little 

 plants out of their own substance while still beneath the surface of 

 the earth, but when the shoots reach up to th^ sunshine this furnishes 

 the energy needed for the work of assimilation and the plant begins 

 to live on the soil and the air. The roots can only send up to the 

 leaf an inorganic sap with possibly here and there an organic cell 

 scattered through it which has penetrated into the roots, as it were, 

 by accident; it is the sunshine that sets these organic cells into 

 activity, causing them to grow and to multiply. 



If a plant in vigorous growth is removed from sunshine to darkness 

 it draws upon its own reserves and lives upon itself as long as pos- 

 sible. In darkness the plant transforms the organic products that 

 are at its disposition, but it can not manufacture any new ones. On 

 the contrary, it consumes itself and its dry weight steadily diminishes. 

 The experiments of Boussingault on seeds, those of Sachs on plants 

 and seeds, those of Pagnoul on the beet, and of Macagno on the grape- 

 vine all confirm this general principle. The observations of the 

 latter show that as between two sets of vines, one exposed to the sun 

 and the other covered with a dark cloth, the growth of the latter, as 

 measured by the amount of solid and gaseous material, was not 10 per 

 cent of the growth of the vine in the sunshine. Other vines under a 

 white cloth showed a growth of 80 per cent, thus apparently proving 

 that the differences were not due to anything else except sunshine. 



Pagnoul experimented upon sugar beets, some of which were cov- 

 ered by glass that had l)een blackened on the inside : this coating of 

 lampblack is ordinarily said to absorb heat, but it would be more 

 proper to say that it transforms all the short waves of the sunshine 

 into long waves so that the plants beneath it receive neither ultra- 

 violet nor visual rays, but only the ultra-red, or long, heat weves. 

 Therefore beneath the black glass the temperature Avas somewhat 

 warmer than beneath the transparent glass and the latter wanner 



