§3] TEMPERATURE-LIMITS OF LIFE 247 



can withstand almost any temperature.* Muller-Thurgau 

 ('80) found that the " succulent labellum of Phajus freezes at 

 — 0.56° C. ; the succulent leaf of Sempervivum, at —0.7°; 

 the potato tuber, at — 1° ; the leaf of Tradescantia mexicana, 

 at — 1.16° ; the ivy leaf, at — 1.5°; the leaves of Pinus austri- 

 aca, at — 3.5°; young shoots of Thujopsis, at — 4°." (Vines.) 

 In this series of plant tissues, we see that the more succulent 

 the tissue, the higher its ultraminimum. Possibly the reason 

 why spermatozoa have so low an ultramaximum, despite their 

 small size, is on account of the denseness of their protoplasm. 



Not all variations in ultraminimum temperature are, however, 

 explicable upon the ground of difference in size, body-covering, 

 or density of plasm. The interpretation of the difference in 

 sensitiveness to cold of the honey bee (Apis melifica) and the 

 red ant (Formica rufa), between Bombyx on the one hand and 

 Smerinthus and Vanessa on the other, must wait for further 

 knowledge. 



The question now arises, what is the cause of death in organ- 

 isms and protoplasm which succumb to low temperatures ? 

 With the higher animals the immediate cause is doubtless in 

 part asphyxia resulting from a stoppage in the flowing of the 

 frozen blood plasma, and in part the destruction of the red 

 blood corpuscles, as well as the white, f With the simpler 

 organisms, like planarians, Protozoa, or Tradescantia hair-cells 

 the case is different. An insight into the changes which pro- 

 duce death in such organisms may be gained from Kuhne's 

 ('64, p. 101) description of the effect of a temperature of — 14° 

 on Tradescantia hair-cells. The frozen hairs were placed in 

 water and observed under the microscope. " The appearance," 



* Striking cases are on record of the resistance of gemmules, or "animal 

 spores," to cold. Thus, Weltner ('93, p. 276) saw gemmules of Spongilla 

 fragilis frozen in an aquarium from December 26 to .January 24, from the end 

 of January to February 5, from February 20 to March 6, and from March 12 

 to 24 ; the intervals being occupied by thawings. Yet these gemmules produced 

 young sponges. In other cases, a certain amount of freezing favors the subse- 

 quent development of gemmules, e.g. those of fresh-water Bryozoa (Braem, '90, 

 p. 8-3) and the eggs of the silk-worm (Dccladx, '71). 



t PoDOHET ('66, p. 18) found that when blood of the frog was let fall into a 

 capsule at a temperature of — 15° few of the red corpuscles were uninjured. In 

 most the nuclei had been cast out into the plasma. 



