284 LECTURE XIII. 



acts : the effect first manifests itself by a diminution of the 

 metabolic activity of the protoplasm, and ultimately effects its 

 disorganisation. 



Plants or parts of plants which have been killed by 

 exposure to high temperatures present the same appearances 

 as those which have been killed by exposure to low tempera- 

 tures. They are flaccid because their cells are incapable of 

 becoming turgid. This is due to the fact that the protoplasm 

 has become permeable as a result of death, and this may or 

 may not be accompanied by its separation from the cell- 

 walls so as to form amorphous masses. 



The foregoing facts suffice to shew that, as in the case of 

 low temperatures so also in the case of high temperatures, the 

 power of endurance is different in different kinds of plants, and, 

 we may add, in individuals of the same species. The figures 

 given above must therefore be regarded as true only with 

 regard to the individual plants or organs experimented upon. 

 A variety of conditions may affect the power which a plant or 

 an organ possesses of enduring exposure to extreme tempera- 

 tures, and it is on account of the influence of these conditions 

 that the results of different observers in any one case are not 

 always quite in accord. For instance, in experiments made 

 with seeds, the seeds, though they may be stated to be dry, 

 are not necessarily equally dry in all cases ; hence the results 

 of different observers as to the extremes of temperature which 

 any particular kind of seed can endure will vary according to 

 the relative dryness of the seeds. Age, too, doubtless exerts 

 an important influence in experiments of this kind : the 

 younger the plant, the more it suffers. The conditions under 

 which the plant had been previously living must also be taken 

 into consideration. G. Haberlandt concludes from his ex- 

 periments that the lower the temperature at which a seed can 

 germinate, the more capable is the seedling of enduring 

 exposure to low temperatures. Still it is possible to base 

 some generalisations upon the ascertained facts. Attention 

 has already been drawn to the most important of these, 

 namely, that the more water an organ contains the more does 

 it suffer in consequence of exposure to extremes of tempera- 



