60 CARNEGIE INSTITUTION OF WASHINGTON. 



Influence of Light upon Growth and Devctopment, by D. T. jIacDougal. 



The author began work on this subject in 1899 and completed an 

 extensive studj^ of the growth and development of plants in darkness 

 in 1903. The conclusions reached at that time included a generahza- 

 tion to the effect that hght does not exert an invariable effect on the 

 rate of growth. The completion of studies of Richards and Spoehr 

 at the Desert Laboratory, upon respiration and acidity, have now 

 yielded some new views and gained some vantage-ground for a new 

 attack upon the subject. 



The organization of this work has required the testing of suitable 

 sources of hght and the construction of a suite of plane and parabolic 

 silvered mirrors by Dr. Ritchey, of the Solar Observatory. The great- 

 est technical difficulty that has been met and overcome, however, has 

 been that of securing screens of glass by which light of certain wave- 

 lengths only might be thrown on the experimental objects. A number 

 of these have been obtained by the cooperation of Dr. H. P. Gage, of 

 the Corning Glass Works, and as soon as testing facilities are organized 

 these screens will be made available to other laboratories. Satisfactory 

 formula have been found for a good monochrome red which removes 

 all light of wave-length shorter than 0.61^ with a refractive index of 

 1.507; a yellow-red transmitting red-yellow and green to 0.53^; a blue 

 transmitting the blue violet from 0.52,a, and hence being complemen- 

 tary to the last; "uviol" transmitting the entire visible spectrum, and 

 a "heat-absorbing glass" which absorbs the infra-red. The last-named 

 has some promise of utility in glazing windows in warm countries. 



The preliminary tests give some indication that the influence of 

 illumination is due to the combined effect of wave-length, total energy, 

 and relation to the absorption spectrum of chlorophyll. The most 

 obvious results are of a morphogenic character, although the rate and 

 amount of growth are doubtless affected. 



The Relation of Soil Aeration to Plant Growth, hij E. E. Free and 

 B. E. Livingston. 



Experiments with Coleus, carried out in the Laboratory of Plant 

 Physiology of the Johns Hopkins University, appear to indicate that 

 this plant requires a certain low rate of oxygen supply to the soil in 

 which it is rooted. The soil-moisture content in these experiments was 

 kept near the physiological optimum and almost constant, by means of 

 auto-irrigators, and a somewhat elaborate system of apparatus allowed 

 the oxygen supply to the soil of the cultures to be cut off and renewed at 

 will. Stems and fohage were exposed to the greenhouse air as usual. 



The first noticeable effect of cutting off the supply of oxygen to the soil 

 was a cessation of water-absorption by the roots, as indicated by the fact 

 that the soil ceased to take water from the irrigators.^ This indication 



^Livingston and Hawkins, Carnegie Inst. Wash. Pub. No. 204, 3-48, 1915. 



