390 



PliOF. TH. W. ENGELMANN. 



Maximum about Ultramaximum about 



Didymium serpula . . 30° C. 



Aethalium septicura . . 39° C. 



Actinosphoerium Eichornii 38° C. 



Miiiola 3S°C. 



Urtica urens 44° C. 



Tradescantia virginica . 40° C. 



Vallisneria spiralis . . 40° C. 



Nitella syncarpa . . . 37° C. 



Chara flexilis — 



35° C. Kiihue. 



40° C. 



43° C. M. Schultze. 

 43°— 48° C. 

 47°— 48° C. 

 47°— 48° C. 



45° C.3 



If the temperature sinks gradually to the minimum, the 

 spontaneous movements stop after they have first become 

 slower and slower. Simplifications of the shape generally 

 occur, and existing processes and branchings are gradually 

 absorbed and new ones are no longer developed. But the com- 

 plex form may remain, under certain circumstances, as observed 

 by Kiihne in Amoeba diffluens (1. c, p. 46) and Actino- 

 sphserium (1. c, p. 68). Optical changes do not generally 

 accompany the entrance into this cold rigor. But Hofmeister"* 

 saw the walls of a protoplasmic mass in Cucurbit a, after 

 standing for a long time at 0° C, become filled with vacuoles 

 and quite frothy. Artificial excitation remains potent, and 

 raising the temperature above the minimum causes a resump- 

 tion of the movements. 



It appears that contractile protoplasm may be kept at the 

 minimum temperature, and even much below it, for an almost 

 unlimited time without sustaining permanent injury. No lower 

 temperature limit, ultraminimum, at which death inevitably 

 ensues, has been described. Even after actual freezing the pro- 

 toplasm when thawed will, under certain circumstances, resume 

 its spontaneous contractions. And in this case it does not even 

 appear necessary that the thawing should take place very 

 slowly, a condition which otherwise is very essential for the 

 revivification of organic structures rich in water. 



' Jiirgersen, ' Stud. d. Physiol. Inst.,' i, p. 104, Breslau, 1861 ; Schultze, &c. 



' Naegeli, 'Beitr. z. wissensch. Bot,,' ii, p. 17. 



3 Dutrochet, ' Compt.' rend.,' 1837, ii, p. 775. 



* W. Hofmeister, ' Die Lehre von der Pflaiizeuzelle,' p. 55. 



