S05 



the death which occurs suddenly as a result of a lowering of temperature 

 below the minimum boundary of heat requirement and which is connected, 

 as a rule, with the formation of ice, may be considered as "freezing to 

 death." 



We can best overcome this difiference in the use of the terms if we 

 consider the first injury^ due to a lack of heat, as a "chronic injury" and 

 sudden death as an "acute injury." 



Tender plants from the tropics, which in our greenhouses do not con- 

 tinuously find the heat necessary for all their developmental phases, often 

 furnish examples of chronic injury. Failures in the culture of Indian 

 varieties of Anoe^tochilus and other tender-leafed orchids. Begonias, Ges- 

 neraceae, Marantaceae, etc., are well known. I found their leaves becom- 

 ing brown-specked, curling and dying if exposed for some time to a tem- 

 perature of 3 degrees above zero to 5 degrees below zero\ In wet, cold 

 years, open ground culture of melons, cucumbers, tobacco and beans became 

 diseased when the lack of heat was prolonged. 



In acute injury, one is inclined involuntarily to ascribe it to the forma- 

 tion of ice. That this in itself does not cause death is shown in many cases 

 , by our hardy plants, which often are frozen stifif and as brittle as glass and 

 yet continue their growth after the frost has disappeared. 



Let us picture to ourselves the effect of the formation of ice in the 

 tissue. If the temperature of the part of the plant has fallen to the freezing 

 point or somewhat below it, small ice crystals are formed on the outside of 

 the cell w^all. These crystals, produced at first from the absorption water 

 and later from the imbibition water of the cell wall, become constantly 

 larger, since, at their base, more and more water from the mycellar inter- 

 stices of the cell wall is changed to ice. Finally, all the fine ice prisms are 

 united into an ice crust. The cell wall has attempted to make up for the 

 loss of water which it has undergone by taking up new amounts from the 

 cell contents. 



Thus the protoplasmic body of the cell becomes poor in water, and 

 material changes begin, which finally reach such an intensity that the 

 equilibrium of the different mycellae of the cell wall and of the proto- 

 plasm is permanently disturbed. They change in such a way that no more 

 life activity is possible. The cell, killed by frost, thus shows that its walls 

 offered no resistance to the pressure of the cell sap, gradually letting it flow 

 away. In direct contact with the air, this passes over into decomposition 

 and the cell itself collapses. The frozen part of the plant appears wilted 

 and dried, or rapidly decays. The cell sap, passing out of it, — this initiates 

 the decay, — presses through the mycellar interstices and not through any 

 breaks in the cell wall which might have been produced by frost. Indeed in 

 a frozen part of the plant the tissue can be blasted by the ice in different 



1 Compare also Molisch, Hans, Das Erfrieren der Pflanzen bei Temperaturen 

 viber dem Eispunkte. Sep. Sitzungsber. d. K. Akad. d. Wiss. Wien. Mat.-naturw. 

 Klasse, Vol. CV, sec. 1; cit. Z. f. Pflanzenkrankh. 1897, p. 23. 



