556 FREEZING AND BURNING. 



plasm alöo. In deserts and steppes, aud in all regions where the eartli is warmed 

 up to 70° in hot rainless seasons, there are, it is well known, a great many annuals. 

 When the hot period commences, leaves, stems, and I'oots are already dead, and 

 the plants have scattered their seeds. These seeds, however, possess very little 

 ■water, and yet can give up a portion of that they contain without injury; they 

 are thus protected in the best way possible against being burnt. 



One portion of the perennial plants of these regions throws off its foliage at 

 the close of the rainy period, and lives through the hot, dry period with leafless 

 and apparently dried-up branches; others expose the whole of their organs above 

 the ground to burning, maintaining themselves below the soil only, where the 

 earth never acquires such a high temperature ; these sleep through the hot period 

 as resting tubers, bulbs, and root-stocks. It should also be remembered here 

 that in regions where higli temperatures are not combined with great dryness, the 

 excessive heat can be diminished by the evaporation from the succulent tissue, 

 since, as is well known, evaporating bodies always undergo a cooling. Finally, 

 the fact is to be considered that many plants choose places for their settlement 

 where they are not exposed to burning, even on the hottest days of the year. 

 Under the protection of shade-giving rocks, and wherever the sun's rays are not able 

 to operate directly and untempered, the soil, even at the equator, may not exceed 

 those temperatures at which succulent plant-organs cannot be burnt, and still less 

 could the normal warmth of the air in shady places bring about such an effect; 

 for the highest temperatures hitherto obser\ed in the shade rise scarcely above 40° 

 (42° in Abu- Arisch, in Arabia; 43'1° on the river Macquarie, in Austi-alia), and at 

 these temperatui'es the albuminous substances are never coagulated in any single 

 plant. 



The question now is how the results obtained from researches into the phenomena 

 of freezing and burning can be brought into harmonj^ with the earlier ascertained 

 relations of heat to living plants, and especially with the theory of growth. We 

 have conceived growth as a form of molecular work of living protoplasm, and we 

 imagine the molecules and groups of molecules to be in a condition of heat-vibration 

 of definite extent; or, in other words, that for all work, and especially for growth, 

 a definite degree of heat is necessary. If the heat-vibrations exceed the fixed 

 limit, the position and the mutual relations of the molecules in the protoplasm are 

 completely altered, and disarrangements result which cannot subsequently be made 

 good. The protoplasm has then lost the capacity of further maintaining itself 

 and increasing — it is burnt and killed. The same happens if the intensity of the 

 heat-vibration sinks below a certain degree. Then again changes are produced 

 in the substance of the protoplasm which are irreparable, and are followed by 

 death. Consequently', a superfluity as well as a want of heat can retard the 

 molecular action of tlie living protoplasm which appears as gi'owth, and can even 

 completely stop it. And the interruption is brought about in the protoplasm of 

 different species under the influence of different degrees of heat; just as water, 

 alcohol, and mei'cur'j- solidify at certain temperatures, and become vaporized at 



