Makch 10, 1916] 



SCIENCE 



361 



Variations of exposure result in variations of re- 

 sponse, even in leaflets of a pair. The xerofotic 

 position decreases the amount of direct radiant 

 energy received per unit area of leaf, reducing the 

 harmful action of too intense sunlight on the 

 chlorophyll, as well as checking the transpiration. 

 Experimental work with screens upon Gliricidia 

 sepiuni; Leucaena gXauca, Mimosa pudioa and 

 Ipomoea pes-caprae limited the causation of the 

 xerofotic response to the action of direct sunlight. 

 The application of chemical drying agents (abso- 

 lute alcohol and xylol) to the upper side of the 

 large pulvini of Gliricidia sepiuni and to the upper 

 side of the midrib of the clam-shaped leaves of 

 the strand plant, Ipomoea pes-caprae, resulted in 

 the assumption of the xerofotic position. 

 The Osmotic Value of Sea Water and the Osmotic 



Surplus of Several Marine Algae: B. H. True. 



The osmotic value of sea water was determined 

 by the plasmolytie method, Spirogyra calibrated 

 by means of NaCl and cane sugar solutions being 

 used as an osmometer. The value so obtained 

 was compared with the values obtained by the 

 freezing-point method and found to agree fairly 

 closely. The osmotic value of several marine algae 

 was also determined and the osmotic surplus was 

 shown to be of the same order of magnitude as 

 that seen in higher land plants. 

 Measurement of the Surface Forces in Soils: 



Chaeles a. Shull. 



The internal forces causing absorption of water 

 by dry Xanthium seeds have been measured and 

 found to have an initial value of at least nine hun- 

 dred and sixty-five atmospheres. The internal 

 forces have been determined at various contents 

 covering the range between air-dry and satura- 

 tion. Dry seeds have been used to measure the 

 surface-holding power of soils for water, with the 

 result that air-dry soils hold water with approxi- 

 mately the same force as an air-dry seed (900- 

 1,000 atms.). As the capillary moisture increases 

 the surface force decreases, until, at the wilting 

 coefficient of the soil, the amount of "back pull" 

 exerted is not more than three or four atmospheres. 

 This relation holds essentially for all types of soil 

 from heavy clay to sand. The soil, therefore, at 

 the critical moisture content for the plant, holds 

 the water with less force than the osmotic pres- 

 sure of the root hairs of plants, as determined by 

 plasmolytie methods. The wilting of the plant at 

 the wilting coefficient does not result from lack of 

 moisture, or lack of a gradient toward the plant, 

 but probably from the low rate of movement of 



water as the friction of movement in thin films 

 increases. 



The Interrelation of Transpiration, Boot Absorp- 

 tion and Water-aisorbing Capacity of Tissues in 

 an Opuntia: Edith B. Shreve. 

 Previous workers have found that the transpir- 

 ing power (i. e., the absolute transpiration rate di- 

 vided by the evaporative power of the air for the 

 same period) is greater in cacti for the night 

 than for the day. The following is a general sum- 

 mary of the results of an investigation into the 

 causes of this phenomenon. (1) The transpiring 

 power is greatly influenced by light intensity, air 

 temperature, water-content of tissues and available 

 soil water; and these factors very clearly exert 

 their influence indirectly through their action on 

 some other internal process. (2) The day- to-night 

 variations in transpiring power are independent 

 of any day-to-night variations in root absorption. 

 (3) During the daylight hours more water is ab- 

 sorbed by the roots than is lost by transpiration, 

 while at night the reverse is true. (4) Variations 

 in water-intake by the roots are due, on the one 

 hand, to variations in soil retentivity and, on the 

 other, to variations of conditions within the plant 

 itself, and the latter may be subdivided into varia- 

 tions in the absolute transpiration rate and in the 

 water-absorbing power of the tissues. The varia- 

 tions in soil retentivity may be reduced to zero by 

 the use of water cultures and of supersaturated 

 soils. Then on dividing the absolute rate of 

 water-intake at the roots by the absolute trans- 

 piration rate for the same period, a quantity is 

 obtained which must represent that part of the 

 root absorption which is independent of varia- 

 tions in transpiration as well as in the retentivity 

 of the soil. This quantity {A/T) has been termed 

 the secondary absorbing power of the roots. Its 

 variations are independent of transpiration, and 

 furthermore, it varies inversely with transpiring 

 power. That is, A/T is greater for the day than 

 for the night and T/E is greater for the night 

 than for the day. (6) Stomata are, in general, 

 shut during the day and open at night, but it is 

 not possible to ascertain whether the closing of 

 the stomata accompanies or follows a decrease in 

 transpiration rate. (7) The water-absorbing ca- 

 pacity of cylinders cut from the internal tissue is 

 less at night than during the day, being least from 

 4 to 5 A.M. and greatest from 3 to 5 p.m. This is 

 true whether the calculations are based on dry 

 weight or on original wet weight. (8) The 

 theory is advanced that the water-absorbing ca- 



