547 



frost does not injure the organs richest in cytoplasm first but, under similar 

 conditions, those poorest in it is a further explanation. 



The grains at the base of the head are, ho\ve\'er, the most poorly nour- 

 ished and poorest in cytoplasm, as may be recognized in any healthy head 

 of grain. 



As a result of a conversation with the director of the German Naval 

 Observatory, Admiral Herz, the latter sent me later, most kindly, the fol- 

 lowing explanation : "In a stand of plants, whether high or low, the soil 

 is, on the one hand, protected against the nocturnal radiation, while, on 

 the other hand, this radiation takes place strongly from the surface of the 

 plantation and, because of poor warmth conductivity, is very effective. 

 However, the air. cooled near the leaves, sifts down through the plantation 

 just as on decli\ities it sinks into depressions in the soil. It is, therefore, 

 very conceivable that the lowest air temperatures begin somewhat below 

 the upper surface of such a plantation, especially if its denseness increases 

 towards the ground, or if the tips are protected from too extensive cooling 

 by a light wind." 



The way in which the processes actually take place out of doors which 

 bring about the injurious cooling of different horizontal air layers at con- 

 siderable distances from the upper surface of the soil is left for further 

 observation. The experiment, in which a w^ooden cylinder, with a mantel 

 containing a cold mixture was set over the upper part of the blossoming 

 rye stalk, pro\cs that the condition of sterile heads is produced by such 

 action of the frost. Because of the impossibility of rapidly mixing the 

 individual horizontal air layers in the freezing cylinder it was proved that 

 only a definite zone was so cooled down that it brought about the described 

 injury to the heads. 



We conclude, for example, from Nordlinger's observation'^ that the 

 injuries take place in forest trees and indicate the existence of a layer of 

 air. which causes death from frost above the warm surface of the soil. 

 Nordlinger found "in June. 1862. in the Hohenheimer Oberen forest young 

 shoots of willow, oak and aspen frozen at the base of the petioles, and in 

 August, 1883, several kinds of willow, especially Salix fraijilis, when there 

 had been no night frost. 



Phenomena of Movement Due to Frost. 



In many plants surviving frost peculiar phenomena of movement result 

 from freezing, which disappear again with thawing. Goppert- mentions 

 Linneus' observation that the leaves of the milkweed {Euphorbia Lathyris) 

 bend their tips backward until the leaf lies against the petiole. The leaves 

 of the wall-flower (Cheranthus Cheiri) look wilted in the frozen condition 

 and often bent, but after thawing they regain their former consistency and 

 position. 



Nordling-er, H., Lehrbuch des Foistschutzes. Berlin, P. Parey 1884, p. 347. 

 Warmeentwicklung in den Pflanzen, p. 12. 



