GROWTH 209 



blue, and black pure-color screens, making all other conditions as similar 

 as possible. Give them favorable conditions for growth, and, when well 

 grown, compare them in all observable features of structure. 



PURE-COLOR SCREENS. These have already been discussed earlier 

 (page 112), and the statements there made as to gelatin or glass screens 

 apply with even greater force to the present experiment. The liquid screen, 

 first spectroscopically tested, may be applied most conveniently by use of 

 the double bell jars of SACHS (figured in DETMER, 28), but a much less expen- 

 sive substitute is afforded by cylindrical jars or bottles (figured by SACHS, 

 "Lectures," 304, or MAcDouGAL, 234) which for the present purpose 

 would have plain cork stoppers. With any of these arrangements ventila- 

 tion should be provided either by aspirator tubes through the stoppers, or 

 by daily renewal of the air, this being done in a dark place. The constant 

 aspirator-ventilation is particularly advantageous because it will tend to 

 offset the somewhat different temperatures which tend to prevail under the 

 different colors. Or the colors can be placed in flat-sided (Soyka) flasks 

 which are laid over flower- pots, in the bottoms of which the seeds are ger- 

 minating, as described in the first edition of this book; but the method gives 

 an insufficient amount of light to the plants, only partially compensated by 

 the use of a mirror to reflect light directly down upon them. 



We come next to the third in importance of the variable exter- 

 nal conditions, moisture. Here it is necessary, obviously, to 

 distinguish between moisture of the air and that of the soil, and 

 we consider, first: 



What effect is produced upon growth by atmospheric humidity? 



This may be determined very simply by keeping the shoots of 

 similar plants enclosed in chambers held at different degrees of humidity, 

 other conditions being the same in all. 



EXPERIMENT. Select 3 single-stemmed plants (e.g., String Beans, 

 Corn) in small pots, and isolate the soil either by inserting the shoots 

 into 3 chambers with .the pots outside (e.g., by aid of supported bell 

 jars, page 187), or else by enclosing the pots in shells and rubber, as 

 for transpiration studies (page 173). Allow the air in one jar to 

 become saturated by the transpiration, in another keep it dry by use 

 of calcium chloride, and in the third keep it on alternate days dry and 

 saturated; give all the plants similar and good conditions for growth. 

 After they have grown for one week, compare the results. 



The effects of varying amounts of soil moisture upon growth 

 are perhaps too well known to require demonstration, but the 

 student may very easily and strikingly convince himself thereon 

 by arranging a dozen pots of Oats or other similar plants, and 

 daily supplying to them different, but to each the same, amounts 



