262 The Living Plant 



up the white Hght are thus injurious, but only the blue-violet, 

 and then only when received in great force; for these very same 

 rays, like some of the red, are the ones that are useful in photo- 

 synthesis. They produce their bad effects, as it seems, through 



their peculiar power of promoting 

 chemical changes, whereby they in- 

 duce in the complicated living 

 protoplasm illegitimate reactions, 

 as it were, which interrupt the or- 

 derly series of chemical processes 

 in which the very life of the 

 protoplasm consists. However, 

 whether this, be the correct ex- 



FiG. 91. — The theoretical form of a de- ■• ,• j. -i. • iU 1 „ 



ciduous tree, consisting of the photo- plauation or uot, it IS nevertheless 



synthetic groundwork shown by fig- ^ f^^^ ^J^^^ strOUg UnSCreeUCd light, 

 ure 7 modified in adaptation to the '^ o / 



mechanical support of the weight of because of its blue rays, is always 



the foliage. ... , i- • , 1 rr-u- 



injurious to nving protoplasm. 1 ms 

 is the reason why bright light is fatal to disease germs, or Bacteria, 

 and explains the basis of the hygienic value of sunlight in the 

 home; while blue light is used with success for the very same reason 

 in the cure of some diseases of the skin. Now because the red 

 rays of the sunlight are not only harmless but also useful, even in 

 fullest intensity, while the blue rays are harmful only when in- 

 tense, but otherwise useful, the problem of adaptive protection 

 against too intense light resolves itself into one of tempering the 

 blue rays without affecting the others. This can be perfectly 

 accomplished through use of a screen which permits red rays to 

 pass while checking the blue, and such a screen is of necessity 

 red. It is upon precisely this principle that photographers use a 

 ruby glass screen in developing their plates, for this color cuts 

 off the blue rays, which are those that took the picture originally 

 and therefore would spoil it in development, while admitting the 

 red rays which are not only harmless to the plate but useful in 

 showing the photographer what he is doing; — only the photog- 



