August 8, 1913] 



SCIENCE 



203 



ing. It was also noted that after a period of 

 rather strong wind the branches drooped much 

 more than was the case in a calm period hav- 

 ing the same temperature. But even such 

 influences failed to prevent the rise of 

 branches on the occurrence of low tempera- 

 ture in case of species which normally raised 

 their branches on the coming of cold weather. 

 It was also found that branches were diverted 

 to the right or left on some trees in proportion 

 to the degree of cold. The branches of linden 

 and those of conifers sank with the tempera- 

 ture, while those of Pterocarya and Acer rose 

 as the temperature became lower. The 

 branches of ^sculus, Oarpinus, Rhamnus and 

 Pavia rose on slight lowering of the tempera- 

 ture and sank when it became colder. The 

 distal ends of the branches on nearly all of 

 the trees under observation stood higher in 

 spring than they did in the preceding fall. 

 The eccentricity of the wood of branches was 

 thought to have no relation to this move- 

 ment, but it seemed that it might be due to a 

 differential contraction and expansion of the 

 upper and under sides of branches, and it was 

 held that this difference in contraction must 

 be distributed over the entire length rather 

 than being confined to the crotch regions. 



Ganong's' observations were more limited. 

 He found that branches move or bend upward 

 or toward the axis as the temperature sinks. 

 He reports that the branches had a greater 

 water content during warmer days of winter 

 than during the colder ones and therefore the 

 thermometric movement. According to the 

 determinations by Geleznow the water con- 

 tent of the wood of Pinus silvestris reached a 

 minimum in June and a maximum in October, 

 while bark has its maximum in October and 

 its minimum in April. Acer platanoides had 

 a maximum water content in the wood in June 

 and a minimum in October; that is, it was 

 found that the minimum water in the wood 

 does not occur in winter, but since his deter- 

 minations were made monthly they throw no 

 light • on the validity of Ganong's inference 

 that the movements depend on periodic varia- 

 tions in the water content. The most recent 



'ioc. cit. 



contribution to this subject is by C. G. Trow- 

 bridge." Although only a summary has ap- 

 peared as yet it promises to be of interest not 

 only because of its content, but also on account 

 of the fact that it is from the physicist's stand- 

 point. Owing to its brevity this summary as 

 given in the proceedings of the Torrey Botan- 

 ical Club is quoted here in full : 



(1) That braneh movements occur in certain 

 trees, due to temperature changes below the freez- 

 ing point of water, and that in certain other trees 

 no movement whatever has been observed. (2) 

 That the movements amount to as much as 3 or 

 4 ft. differences in the distance from the ground 

 to the ends of certain curved branches which are 

 in length of the order of 20 ft., these changes 

 occurring through a range of 30 degrees below 

 freezing. (3) That little, if any, movement takes 

 place above freezing point of water, and that the 

 movements begin soon after the temperature re- 

 mains at this point for several hours. (4) That 

 there is a considerable lag in the movement of the 

 branches behind the temperature changes, although 

 a difference in the rate of change of temperature 

 is followed at once by a difference in the rate of 

 change of the position of the branches. (5) That 

 the movements are practically of equal magnitude 

 in December, January and February, that is, the 

 seasonal change is not a ruling factor in this 

 movement. 



According to Geleznow, then, tree branches 

 may move either up or down as the tempera- 

 ture sinks. He found that eccentricity of the 

 wood is not correlated with this movement, but 

 that a difference in the water content of the 

 wood on the upper and under sides of branches 

 seems to be, yet he did not consider that an 

 explanation of the movements but only a 

 suggestive parallel. Caspary found three 

 classes of trees in regard to the manner of 

 branch movements: In one class the branches 

 sink and in another they rise on lowering of 

 the temperature and in the third class the 

 branches rise as the temperature is lowered 

 slightly but sink when it gets still colder. 

 According to him the movements of branches 

 result from a differential contraction of the 



' ' ' Branch Movements of Certain Trees in 

 Freezing Temperatures," Torreya, 13: 86-87, 

 1913. 



