554 FREEZING AND BURNING. 



and, in order that dry starch -grains may be destroyed, a still higher temperature 

 is necessary. Under such conditions it is not to be wondered at that plants, 

 whose protoplasm exhibits different coagulation points, should be "burnt" at very 

 different temperatures. The processes which have been observed in the above- 

 mentioned Elodea at 30°, 41°, and 52°, are seen to occur in other water-plants at 

 other temperatures. In the cells of Vallisneria spiralis, represented in fig. 52^ 

 the streaming of the protoplasm does not stop till 43° has been reached, and the 

 protoplasm is not formed into balls in consequence of the coagulation of the 

 albumen till 53°-54°. In the lattice-leaved Aponogeton fenestrate, growing in 

 Madagascar, the coagulation and death of the protoplasm first occur at 55°. Many 

 algse bear even still higher temperatures. In the channels through which the hot 

 water of the Carlsbad spring flows, dusky oscillarias flourish even at a tempera- 

 ture of 55° to 56°; in the springs of Abano, which reach a temperature of nearly 

 60°, SphoRrotilus thermalis is to be found, and in the Solfatara at Naples, the 

 side-walls of the rocky clefts, from which vapour issues at a temperature of 55° 

 to 60°, are covered with a green film of algse. 



In plants which are not submerged in water, the watery contents as well as the 

 specific constitution of the protoplasm have a material influence on the burning. 

 If the exposed tissues are poor in water they can sustain much higher tem- 

 peratures than when very turgid. The highest temperature which the turgid 

 cells of lithophytes and land plants can endure without being burnt is in most 

 cases 55°. In the sun, succulent plants can endure for a long time without injury 

 temperatures of 50° to 53°. The spores of moulds {JRhizopus nigricans and Peni- 

 cillium glaucum) have been seen to germinate and develop at from 54° to 55°. 

 When dry, those cells and tissues which can be dried up without harm do not 

 perish even under the influence of far higher temperatures. The crustaeeous 

 lichens adhering to the limestone rocks of the wild regions of the Karst of Istria 

 and Dalmatia (Asjjicilia calcarea, Verrucaria purpurascens, and V. calciseda) 

 are regularly exposed on cloudless summer days to a temperature of 58° to 60° 

 without injury, and the edible lichen {Lecanora esculenta), illustrated opposite, is 

 often heated in the deserts, along with the stone on which it grows, to fully 70°, 

 and yet is not destroyed. Moreover, seeds which are deposited on the desert 

 sands, and survive in this position the long periods of drought, do certainly 

 assume the temperature of their environment, and although at noon this often 

 amounts to 60°-70°, it does not injure these seeds; since, when the rainy season 

 returns, they are roused from their summer sleep, and germinate in the cool and 

 moistened soil. The highest temperature in the superficial layer of soil has been 

 observed near the equator at Chinchoxo on the Loango coast. Here, in many cases 

 it exceeds 75°, often attains 80°, and once attained to even 84-6°. Nor is this soil 

 destitute of annuals during the rainy season, and without doubt the dry seeds of 

 these plants have been lying for months in the sand, sometimes heated to over 

 80°, without losing their germinating power. It has been proved experimentally 

 that seeds, which have been deprived by calcium chloride of as much water as 



