FREEZING AND BURNING. 563 



cooling, and that the most effective protection must be sought for in the constitution 

 of the protoplasm itself. Since we do not know the constitution of the protoplasm, 

 it is idle to puzzle ourselves in surmises about it; this only being certain, that the 

 resisting capacity of protoplasm differs much from plant to plant, as well as at 

 different times in one and the same plant. 



The results which have been obtained by the study of the burning of plants are 

 analogous to those afforded by researches into the nature of freezing. 



When a plant organ loses its capacity of absorbing food, of breathing, and of 

 further development, in consequence of the rise of temperature, we say then that 

 it is burnt. The outwardly visible appearances of burnt plants resemble exactly 

 those which have been observed in plants killed by freezing; the green tissue is 

 discoloured, exhibits a darker tint, is more transparent, fades and dries up, and 

 neither the supply of water nor the reduction of the temperature can reproduce 

 the previous conditions. The protoplasm in the interior of the cells is massed 

 into balls, and is detached from the cell-wall; water is excreted, which had 

 stood hitherto in molecular combination with the protoplasm. These observations 

 can be followed very easily in aquatic plants whose cell-walls are so transparent 

 that they allow us to see into the interior of the cell-chambers. If the cells of 

 the water-plant Elodea, illustrated in fig. 5 ^ (page 25), are examined under the 

 microscope while the temperature of the surrounding water is 30° C, the proto- 

 plasm will be seen to exhibit that active streaming movement described on p. 33. 

 If the temperature is raised to 40°, the streaming becomes slower, and at 41° 

 ceases entirely, although the protoplasm shows no other particular alteration. 

 Even if the temperature is raised to 45°, and gradually to 50°, nothing is altered 

 in appearance; not until 52° does any very noticeable alteration occur. Then the 

 starch-granules imbedded in the protoplasm split up; the protoplasm shrinks 

 together and forms clump-like masses around the fractured starch-granules. The 

 protoplasm now becomes rigid, the albuminous materials in it are curdled or 

 coagulated. Subsequently, if the temperature again sinks to 30°, it does not 

 become again living and active, and we must therefore assume that its molecular 

 constitution has suffered at 52° an irreparable alteration, in fact, that it has been 

 killed. 



In the main, therefore, burning depends upon the coagulation of the albumi- 

 nous compounds, upon the destruction of the starch-granules, and the decom- 

 position of the protoplasm. If the coagulation of the albuminous compounds and 

 the alteration of the starch-granules were always brought about by one and the 

 same temperature then probably all plants would be " burnt " at this same tem- 

 perature. But such is not the case. The various albumens not only coagulate at 

 different temperatures (viz. 60°-80°), but the point of coagulation of the same 

 albumen is materially affected by the watery contents, and by the presence of salts 

 and acids. When, for example, many salts are present, coagulation may occur at 

 50°. Nor does the destruction of the starch-granules always occur at the same tem- 

 perature; large starch-grains, swollen with water, at 55°, smaller ones not tiU 65°; 



