NO. 14 EFFECTS OF INFRA-RED OX PLANTS — JOHNSTON 3 



method of making this iron compound has been described and dis- 

 cussed by Burk, Lineweaver, and Horner (6). This compound 

 promises to be a very useful source of iron for nutrient sokition ex- 

 periments. UnHke most of the other iron compounds used in this 

 type of work, the sokition contains very httle if any precipitate, 

 even at high pH vakies. Hence it is not necessary to add this form 

 of iron every day or two during the early stages of growth as must 

 be done with ferric tartrate and some other iron compounds. One 

 application of iron humate is sufficient to keep the plants green 

 under good growing conditions for at least two weeks. 



EXPERIMENTAL PROCEDURE 



In the experiments herein described, only the overhead illumination 

 was used. The light period for each 24 hours extended from 9 a. m. 

 to 12 midnight. This period of illumination (15 hours) falls within 

 the optimum range for tomato plants. Two wave-length ranges for 

 two different light intensities were used. One wave-length range in- 

 cluded all radiation transmitted by a water cell 1.5 cm thick with 

 pyrex cover glasses, and the other was further limited by a heat- 

 absorbing filter. Two chambers of each pair had the same visual 

 intensity. It is realized, of course, that the radiant energy required 

 in plant reactions is not exactly limited to the visible region, nor 

 is it at all likely that their requirements are at all proportional to the 

 visibility. It would be preferable to compare the effect of radiation 

 in the range absorbed by chlorophyll with radiation including the 

 near infra-red as well. Practical considerations make it necessary to 

 use a heat-absorbing fdter which cuts off not sharply at the limit of 

 chlorophyll absorption, but gradually from 6,000 to 8,000 A. The 

 method to be described for equalizing the visual intensities is simply 

 a convenient means of attaining approximate equality of intensities 

 in the visible range common to both types of radiation. 



The light intensities were equalized at the beginning of the ex- 

 periment by means of a Weston photronic cell provided with a special 

 heat-absorbing filter (Corning heat-resisting, heat-absorbing, dark 

 shade filter 2.82 mm thick). At the conclusion of the experiment of 

 two weeks' duration the light intensities gave the values indicated 

 in Table i. This combination yielded a sensitivity curve shown as a 

 continuous line in Figure i. Sensitivity is plotted as ordinates in arbi- 

 trary units with 100 as maximum against wave lengths in Angstrom 

 units. The visibility curve shown as the dash line is included in this 

 figure for the sake of comparison. 



