1894. ] Crystals of Ice on Plants. 325 
a. Conditions of formation of crystals. 
From what has been said it appears that the conditions for 
the formation of the ice-crystals are: 
1. The temperature must be above zero in the soil sur- 
rounding the roots of the plant [Le Conte and Sachs]. 
2. Ihe temperature of the central parts of the water-con- 
ducting tissues must be above zero. 
3. The soil must be sufficiently charged with water. 
Summary. 
The cold causes a contraction of the tissues all over the 
Sues, and it may be assumed that different plants (even differ- 
ent individuals of the same species or different parts of the 
Same plant) have a different coefficient. MacDougal supposes 
that plants with large and numerous vessels and thin-walled 
cells in the medullary rays are especially adapted to form 
sheet-crystals. It is quite sure that such plants have a larger 
Coefficient of contraction than other plants. 
Capitol, Des Moines, Towa. 
; [Since the above article was written, I have found a quite 
interesting note entitled “Frost, and the Cunila Mariana L., 
= dittany,” by J. Stauffer, Mount Joy, Penn’a, in The Horti- 
Culturist and Journal of Rural Art, 7: 73-74. New York, 
1857. On December 6, 1856, Mr. S. was passing near the 
Willistown Baptist Meeting House, in Chester co., Pa. 
ere, he found the ice crystals: ‘‘What, however, amply re 
Compensed me for the attention bestowed, was the discovery 
