549 



sometimes the morphologic upper side, sometimes the under side of the joint 

 cushion is shortened. Among the changes appearing especially clearly with 

 ice formation, the curling of the leaf surfaces should be emphasized. An 

 example very easily observed is furnished by our Rhododendron. Harsh- 

 berger^ describes a case of Rhododendron maximum in which the petioles 

 sank to 70 degrees while the edges of the leaf curled backward so much 

 that the upper surface appeared convex. If the plants were brought into a 

 warm room, their leaves resumed their normal position after 5 minutes. 

 Hartig ascribes this process to a peculiar irritability of the cytoplasm while 

 I assume diiferences of tension between the differently constructed layers 

 of tissue. 



In many woody plants a movement of the branches and twigs propor- 

 tionate to the degree of cold is found. According to Caspary- Acer 

 negundo and Pterocarya caucasica direct their branches upward, while 

 Larix, Pinus S'trobus and Tilia parvifolia lower their branches. Aesculus 

 Hippocastanum and Aesculus Hippocastanum rubra, as well as Car pinus 

 Behdus lower their branches with a slight degree of frost and raise them 

 again when the cold becomes greater. Simultaneous with this raising and 

 lowering is a lateral motion, in some varieties toward the right, in others 

 toward the left. In Cornus sanguinea Frank'' found that the one to three 

 year old branchlets became wavy and twisted above each other. Most of the 

 twistings were found to be clearly directed toward one and the same point 

 of the compass so that Frank concluded it was the effect of a current of 

 cold air coming from a certain direction. 



As stated above, we might seek the causes for the movements in leaves 

 and petioles, as well as in branches, in dift'erences of tension which takes 

 place, partly because of changes in turgidity, partly from unecjual contrac- 

 tion of different tissue forms within the same organs due to the appearance 

 of cold. 



An experiment which I carried out with Aesculus Hippocastanum 

 proves that an increase of turgidity of the parenchymatous tissues in "leaf 

 zvilting due to frost" can. inider certain circumstances, again cause the stif- 

 fening of those leaves. 



A three year old potted specimen was put into a warm place in Febru- 

 ary. It developed very vigorously until the middle of March so that the 

 terminal shoot, 14 cm. long, had developed six leaves. The largest leaflet 

 of the two youngest leaves was 2.5 cm. long and in the lower, older leaves 

 the length of the petiole was from 5 to 9 cm. 



The plant was put out of doors on March 14th. The following night 

 the temperature fell to 2.5 degrees C. below zero, and the next morning a 

 sharp bending or breaking of the petioles was noticed on four of the older 



1 Harshberger, .John, Thermotrophic movements of the leaves of Rhododen- 

 dron maximum; eit. Bot. Jahresb. 1899, II, p. 141. 



- Report of the International Horticultural KxhiV)ition, etc., I^ondon 1S66; cit. 

 in Niirdlinger, Forstbotanik, I, p. 201. 



3 Frank, A. B., Krankheiten d. Pflanzen. Breslau \^^:^, v. 1, p. 187. 



