DEPARTMENT OF BOTANICAL RESEARCH. 59 



constancy, increase, or decrease of illumination, even when this has 

 been increased with respect to the growing part by insolation from three 

 directions. There seems, indeed, to be no maximum insolation nor- 

 mally occurring in the field at this locality which can cause any cessa- 

 tion or inhibition of growth when conditions obtain which insure 

 water-supply to the growing part. Thus, when a cessation of growth 

 is apparent, it may be checked, and high rates instituted, by the 

 removal of leaves (which divert the water-supply), by increasing the 

 vapor-tension in the vicinity of the growing part, or by merely increas- 

 ing the temperature when the volume of the growing part is small (as 

 when the internode under observation is young). These positive 

 changes may occur coincidentally with increase of illumination from 

 the blue or red portions of the spectrum to full insolation. 



It thus appears that the dominant factor in the mechanism of growth 

 is the water-balance of the growing part, a conclusion in accord with 

 studies of A. M. Smith, Lock, W. L. Balls, and Blaauw, thus substan- 

 tiating (contrary to the widely diffused belief) the view earlier pro- 

 nounced by MacDougal that light under normal conditions does not 

 inhibit growth. 



Glass Screens for testing the Influence of Light upon Growth and Development, 



by D. T. MacDougal. 



Glasses for the purpose of screening organisms from various parts 

 of the solar spectrum have been fixed upon by cooperation with 

 Dr. H. P. Gage, of the Corning Glass Works. The manufacture of 

 this material was at first confined to the molding of bell-jars. Not 

 all of the desired ones could be treated in this manner, and during the 

 present year the effort was made to blow cylinders which could be 

 flattened and annealed into sheets. This process has also proved 

 unsatisfactory and has been discontinued. The method finally fixed 

 upon is that of stamping out small sheets 6.5 by 6.5 inches (about 16 

 cm. square). Some glasses in this form have already been secured and 

 others are in course of manufacture. 



The Distensive Forces in Growth, hy D. T. MacDougal. 



The embryonic cells of the growing regions of plants, to the activities 

 of which the external measurable features of growth are due, are com- 

 pressed globular masses of colloids of varying composition and disper- 

 sion, including both nitrogenous gels and mucilaginous material of the 

 pentosans and hexoses. The outermost layer of each protoplast is the 

 seat of speciaHzed activities and is of greater density than the re- 

 mainder of the unit. Not all solutes or suspensions pass through it 

 with equal facility. The kinetic theory of osmosis, by which a pres- 

 sure is set up inside this membrane by the impact of the molecules 



