324 EXPERIMENT STATION RECORD. 



In the investigations on tlie influence of pliysical factors on vegetation 4 

 -stations have l)een located, one at the Desert Laboratory, a second on a small 

 UTigateil farm, a third at an elevation of G,K)0 feet, and the fourth an alpine 

 station at 8,000 feet. At these different stations observations are being carried 

 on in which measurements are made of the factors which go to make up the 

 climate of a region. It has been determined by the methods pursued that a 

 meadow received 7S,S36 hour-degree units of heat in a year, while the floor of a 

 hemlock forest near by received but 68,596 hour-degree units, indicating a 

 season practically 11 days shorter than that in the meadow. In the application 

 of this method to plants having a resting season the exposure is calculated 

 from the time of ])lanting the seeds until their maturity or until the attainment 

 of some stage of development. 



A number of correlated problems are under observation, some of which will 

 be reported upon in the publications of the institution. 



A new form of automatic rain meter is figured and described. 



The physiological constant for the germinating' stage of cress, F. A. 

 Waugh and C. S. Pomeroy (Massachusetts Sta. Rpt. 1907, pp. 71-80, dgms. 3). — 

 A study was made of the germination of cress to determine the physiological 

 constant or the amount of heat required for the germination of the see<l of this 

 plant. These investigations have been carried on for some time at the station, 

 and the authors have employed n recording thermograph in order to secure 

 greater accuracy in the data. 



During the period reported upon, 77 thermograph records were obtained 

 showing the sum temperatures above 32°. In these records the sum tempei'a- 

 tures varied from 2,714 to 4,286, and the time occupied for the completion of 

 the stage of germination from 70 to 210 hours. By means of formulas the 

 authors have determined the critical temperature of the germination phase of 

 cress to be 38.2°. With this temperature known, the germination period of 

 cress can be computed within rather close limits. 



The transpiration current in plants, J. M. Janse {Jahrh. Wiss. Bot. 

 [Pringsheim], 7/5 (1908), No. 3, pp. 305-350, figs. 13).— The author has made a 

 study of the movement of water in plants, paying particular attention to trans- 

 piration. He claims that the movement of water in plants in so far as it is 

 caused by transpiration is to be regarded exclusively as an isothermic operation 

 for which all the energy is furnished by the temperature of the environment. 

 If the leaf cells assist in moving the transpiration current, their effect is so 

 small as to be only theoretical. 



Molecular activities, such as capillary, imbibition, cohesion, etc., are inca- 

 pable of promoting the water movement. 



The course of the transpiration current is through the vessels when they 

 conduct water only, the spiral vessels, even when containing air, and through 

 the tracheids which are filled with water or which may contain air bubbles. 

 In the case of the tracheids containing air bubbles, the water passes upward 

 between the wall and the bubble, and it is probable that it filters sidewise into 

 the next higJier tracheid. Where the vessels are contiguous to the tracheids 

 the water contained in them may enter the transpiratory current even if air 

 bubbles are present, but then only by sidewise filtration from the vessels into 

 the tracheids. 



Where the vessels are separated from the tracheids by wood-parenchyma 

 cells the water held in them can not enter the transpiratory stream directly, 

 but it seems probable that it can act as a reserve supply when transpiration is 

 suddenly increased. 



The resistance encountered by the transpiratory stream under these condi- 

 tions is so great that water can be moved for only relatively short distances. 



