49 



when all the cells are killed, is then in the neii>-lil)orhood 

 of -5°. The lethal temperatures extend, therefore, from 

 -2° to -5°. (Unpnhlished data; paper presented before 

 the Indianapolis meeting- of the Bot. Soc. of America, 

 Dec. 1937.) 



4. Tissue from Leaves, Stems, Roots, etc. In this sec- 

 tion, we shall deal with plant organs rather than with plant 

 tissues. In some cases the investigators, in their studies 

 of organs, have endeavored to tind which tissues were 

 involved, but most of the time we have no such information. 



The agricultural and horticultural literature is rich in 

 observations made in the open on death temperatures of 

 leaves, buds, flowers or stems of various species of plants. 

 Tables of observational and also of experimental data 

 have been published. The authors of most of these tables, 

 however, intended primarily to furnish the agriculturists 

 with instructions easy to follow, and they do not give such 

 factors as the time during which a plant organ can be 

 exposed to a given degree of cold or the internal tempera- 

 ture of the organ. Instead of quoting these survey tables, 

 we think it, therefore, more useful for our purpose to 

 describe some of the more significant experiments. 



The leaves of some plants are killed by the action of 

 temperatures above zero. Molisch (1897) exposed the 

 isolated leaves of Episcia to temperatures of 2.5° to 4.4°. 

 After 24 hours, several leaves became bro^^i and spotted 

 and after 4 days, they were all brown. The plasmolysis 

 test revealed that the cells were dead. On another experi- 

 ment, the leaves were put in ice water between 0° and 1°. 

 In three hours, they exhibited brown spots and in 24 hours 

 they were entirely brown. 



Miiller-Thurgau (1880), using as a death criterion the 

 change in color that the petals of some orchids undergo 

 during protoplasmic disintegration, put to freeze in a 

 cooling chamber petals of Phajus graudifolius wrapped 

 around the bulb of a thermometer. The temperature sank 

 first to a subcooling point, then rose suddenly to the freez- 

 ing point, stayed there for a short time and finally dropped 



