20 8 PLANT PHYSIOLOGY 



all the way from o to near 15, with a mean at about 5; hence 

 the conventional minimum would be expressed as o-5 -i5. Similarly 

 the optimum ranges from below 25 to about 35, with a mean at about 30, 

 whence the conventional optimum of 25-3O-35. The maximum ranges 

 from below 30 to somewhat over 45, with a mean at about 40, whence 

 the conventional maximum is 3o-4O-45. The conventional expression 

 for the three cardinal points of plants commonly used in the experimental 

 laboratory would be 5-30-4o. 



So much for temperature. We" turn next to consider the 

 second in prominence of the variable factors to which living 

 plants are exposed, Light, and proceed to ascertain: 



What effect is produced upon growth by light? 



EXPERIMENT. Prepare 3 two-inch pots of Oats, 3 of String Beans, and 

 3 of Tropaolum, or Morning-glory, as for the preceding experiment. Place 

 them in 3 groups, each of the 3 kinds, side by side on a movable tray, and 

 cover each group by a closed bell jar. Cover one bell jar by an opaque 

 hood, one by a cloth hood giving about half light, and leave one uncovered; 

 place them all in strong diffused light. Give them daily aeration and such 

 water as they need, uncovering them only in a darkened place; and, after 

 the seedlings are well grown, compare them as to average height, thickness, 

 color, wealth of leaf, and other observable features. 



The student should now extend his knowledge of this sub- 

 ject by observation of cases of light-and-dark effects occurring 

 naturally, and by study of the books, and should make himself 

 acquainted with present knowledge of the important subject of 

 the conditions 0} growth in darkness, with phenomena of etiola- 

 tion, and also the various kinds of effects produced by light upon 

 growth. In this connection he should make acquaintance with 

 MACDOUGAL'S very important work, "The Influence of Light 

 and Darkness upon Growth and Development" (Memoirs of 

 the New York Botanical Garden, 2, 1903). 



It will now occur to the student that light is so composite 

 a source of energy that it will be necessary to determine to which 

 of its constituents the resultant effects are due. The proper 

 experimental study of the subject is of considerable practical 

 difficulty, but if the student can undertake it, he will be aided 

 by the following: 



SUGGESTED EXPERIMENTS. Prepare 5 small pots of Oats as for the 

 preceding experiment, and place them under, respectively, white, red, green, 



