190 BOTANY. 



remaining solution made denser still. These adhesive forces, 

 tend to retard the formation of ice-crystals, and it is prob- 

 able that it is only in extremely low temperatures, if at all, 

 that the liquids in the plant are completely solidified. 



248. — A plant which has been frozen may survive in many 

 instances if thawed slowly, whereas if thawed quickly its 

 vitality is generally destroyed. Thus many herbaceous 

 plants will endure quite severe freezing if they are afterward 

 covered so as to secure a slow rise of the temperature, and 

 many bulbs, tubers, and roots will survive the severest win- 

 ters if covered deeply enough to prevent sudden thawing. 

 Likewise turgid tissues, which are not living, as those of 

 many succulent fruits, are injured or not by freezing, accord- 

 ing as the thawing has been rapid or slow. From these facts 

 it may be inferred that the injury in freezing is primarily of 

 a physical instead of a chemical nature, and that it is mainly 

 the withdrawal of water from its physical union with the 

 solids of the cell. According to this view, the difference be- 

 tween slow and rapid thawing is that in the former the 

 slowly liquefying water is reabsorbed by the same solids from 

 which it had been abstracted, while in the latter the large 

 amount of water set free is imperfectly absorbed, forming 

 solutions which are unstable and subject to subsequent fer- 

 mentive changes. It is probable that to these fermentive 

 changes is due the coagulation of the albuminoids and 

 the rapid disorganization of the protoplasm which accom- 

 pany injury from freezing. 



While the sketch given above is doubtless true in a large 

 number of cases, it appears that in many other cases death 

 follows freezing whether the thawing be rapid or not ; and 

 this indicates that besides the immediate causes of death al- 

 ready indicated, there are others which are as yet unknown 

 to us. 



§ II. Light. 



249.— General Relations. Directly or indirectly plant- 

 life, as indeed all life, whether vegetable or animal, is de- 

 pendent upon light. Parasites and saprophytes may grow 



