518 SOURCES OF HEAT. TRANSFORMATION OF LIGHT INTO HEAT. 



animals and in various other instances of slow and quick combustion of organic 

 bodies, which the growing plants can now and then directly utilize. These, how- 

 ever, are only derived sources of heat. Heat which is liberated in respiration is 

 really only the sun's rays which the plants have absorbed on a previous occasion, 

 and ultimately, so far as it comes under consideration for the life of plants, all heat 

 is derived from the sun. The heat which is conducted to plants from the soil, from 

 water, and from air, also takes its origin from the sun, which is therefore to be 

 looked upon as the fountain-head of all the heat utilized by plants. 



It has been found that the sun sends out three kinds of rays distinguished by 

 their different periods of vibration, and known respectively as heat rays, light rays, 

 and chemical rays. These three undulating movements of the ether interfere with 

 each other in their course as little as the wave-circles which intersect on the surface 

 of water. We recognize and measure them by their effects. As soon as they strike 

 a body, work is performed by the active force of these ether waves which we picture 

 to ourselves as movements of the molecules and atoms of the body affected; and this 

 work appears either as heat, or light, or chemical change. But it is exceedingly 

 remarkable that only that movement which we regard as heat can produce that 

 transformation of building substances into organized materials, which is, in other 

 words, growth. The vibrations which constitute light, and whose great importance 

 in the formation of the constructive materials, and generally, of organic compounds 

 from inorganic food, has been previously described in detail, are not able to cause 

 such an effect, at least directly. There are even instances which justify the opinion 

 that growth is actually restricted and hindered by light. This much is certain, 

 that growth can proceed in the deepest gloom, if only the two earlier-mentioned 

 factors turgidity and heat are undiminished. Seeds and the majority of spores 

 germinate in darkness. The cells of underground stems and scale leaves, those of 

 roots embedded deep under the soil, as well as the mycelia of fungi, grow in regions 

 wholly deprived of light. Moreover, plant organs which are brought from the light 

 into darkness continue to grow there, provided always that the necessary amount 

 of moisture and heat be supplied to them. 



Nevertheless very numerous experiments tend to prove that growth can be 

 assisted by light. The following is one of the most remarkable. If plants are 

 cultivated in two places, identical as to the amount of heat affecting them during 

 growth, but differing in the intensity and duration of the flow of light, they will 

 exhibit a quicker growth in the place where the light can act on them more power- 

 fully and for a longer time. Thus plants grow much more quickly in the far north, 

 where they are daily illuminated for twenty hours, than in southern latitudes 

 where they are exposed to the light for only twelve hours, even although in the 

 same space of time comparatively less heat reaches them in their northern habitat. 

 From the small table inserted opposite, giving the commencements of the flowering 

 periods best adapted for the comparison of a definite amount of growth in 

 several widely-distributed plants, at Athens, Vienna, and Christiania, it may be 

 seen that Athens is about forty-six days earlier than Vienna, but Vienna only 



