SOURCES OF HEAT. TRÄNSFORMATION OF LIGHT INTO HEAT. 517 



through a metre, and iu still larger forest-trees the figures become even more 

 impressive. 



And all this is accomplished by the invisible atoms of the living protoplasm, 

 which, set in motion by heat, alter their position, attract and repel one another, 

 dispilace and travel between one another, assume new groupings, and in these new 

 arrangements appear outwardly to our senses in altered form and increased volume. 



On glancing over these effects of growing cells and groups of cells, one is 

 reminded involuntarily of the analogous phenomena of ice crystallization. When 

 ice is formed in a glass bottle filled witli water, it bursts the vessel with irresistible 

 force, and the splitting of masses of rock in high mountains and in all those regions 

 where the temperature in winter sinks below freezing-point depends in no small 

 degree on the freezing of the water which "has penetrated into the smallest crevices 

 and rocky clefts. And yet there is an essential difference between growth and 

 crystallization. Crystals are formed spontaneously from fluid substances, and 

 grow from the depositions of small atoms on their surface. Vegetable cells, on 

 the other hand, never arise spontaneously from fluid materials, but always only by 

 means of an already present organized and living mass of protoplasm. Thus all 

 growth in living things is reallj' only a further development of what already exists. 

 The crystal can again be transformed into a formless fluid mass, can be reconstructed 

 from this fluid, and this alternation may be repeated innumerable times. In plants, 

 on the other hand, the passage from the formed, organized, to the formless, fluid 

 condition is synonymous with death, and from the gases and fluids which are 

 derived from the decomposition of a vegetable-cell a plant-cell never again forms 

 itself spontaneously, that is, without the interposition of a living agent. While, as 

 above remarked, crystals grow by the deposition of small particles on their surface, 

 growth of protoplasm takes place by the interpolation of new molecules between 

 those already present: these are separated from one another, and only subsequently 

 can parts of the cell increase by deposition brought about by living protoplasm. 



2. GROWTH AND HEAT. 



Sources of Heat — Transformation of Light into Heat. — Influence of Heat on the Configuration and 

 Distribution of Plants. — Measures which protect Growing Plants from Loss of Heat. — Freezing 

 and Burning. — Estimation of the Heat necessary for Growth. 



SOURCES OF HEAT. TRANSFORMATION OF LIGHT INTO HEAT. 



Whence do plants derive the heat necessary for their growth? With regard to 

 this question one may first of all think of that heat which is liberated by the plant 

 itself in respiration, and which can again find employment immediately after its 

 release, not only in metabolism and transport of materials, but also in growth. 

 Further, we may be i-eminded of that heat which is liberated by the breathing of 



