186 BOTANY. 



plants the lower temperature limit is apparently somewhat 

 higher than in aerial ones ; thus in Hottonia palustris it is 

 2.7 Cent. (37 Fahr.) ; in Vallisneria, 6 Cent., or more (42 

 Fahr.) ; in Potamoqeton from 10 to 15 Cent. (50 to 59 a 

 Fahr.). 



Neither the maximum nor the optimum temperature has 

 been determined for ordinary land plants; in Hottonia 

 palustris, an aquatic plant, the maximum temperature for 

 assimilation is, according to Sachs, between 50 and 56* 

 Cent. (122 and 132 Fahr.). 



245. Metastasis. But little is accurately known as to 

 the effect of an increase or decrease of temperature, within, 

 moderate ranges, upon those metastatic changes which take 

 place in the ordinary growth of plants or the storing of reserve- 

 material. It is well known, however, that some plants live 

 wholly in low temperatures, performing all their functions 

 in air or water little, if any, above the freezing point. 

 Thus in the " Eed-snow Plant," above cited, the metas- 

 tatic changes must take place very near Cent. 



In the polar waters, where the temperature is from 3 to 

 5 Cent. (37 to 41 Fahr.), or even less, myriads of diatoms 

 flourish, and in seas but little warmer many of the higher 

 sea- weeds (Fucaceae and Florideae) abound. In all these 

 cases the metastatic changes (as well as all others) must take 

 place at these low temperatures. In ordinary land-plants it 

 is to be observed that whereas assimilation takes place only 

 during the light part of the day, when it is warmer, metasta- 

 sis takes place not only in daylight, but even more rapidly in. 

 darkness, when the temperature is considerably lower.* 



Sachs measured the length of plumule developed upon 

 different plants of the same species subjected to different 

 temperatures, and in this way found the approximate optima, 

 for several species, as follows :f 



* It must not be forgotten, however, that assimilation is dependent 

 upon light, while metastasis is somewhat checked by it, and this is 

 doubtless by far the most important relation ; and still it is a significant 

 fact that in ordinary land-plants metastasis continues when assimi- 

 lation has stopped. 



fin " Physiologische Untersuchungen iiber die Abhangigkeit der 



