320 GROWTH OF PLANTS 



From Popp's results it is evident that the ultraviolet of sunlight has 

 Httle part in determining the stature, form, and flowering of plants, whereas 

 the blue-violet is very important in determining form. Ai'thur ^ says there 

 is no marked difference in form, dry weight, or tune and amount of flowering 

 whether plants receive only the ultraviolet transmitted by \vindow glass 

 or receive in addition the shorter ultraviolet of sunlight. Shirley ^^ speaks of 

 the entire spectrum, visible and ultraviolet, as more important for the growth 

 of plants than any portion of the spectrum, and of the blue region of the 

 spectrum as more important than the red. Whatever the significance of 

 ultraviolet on the form, stature, and flowering of plants, it has a number 

 of other interesting effects, as we shall see in a subsequent section of this 

 chapter. For the later results artificial sources of ultraviolet were used 

 with filters, to give control of both range of wave length and intensity. 



Some Effects of Ultraviolet Rays on Plants 



The formation of anthocyanins in plants may take place in darkness, as 

 is the case with beet roots; it may be hastened by light, as in cranberry 

 and Abundance plum; or it may be formed only in light and only in the 

 cells actually exposed to the light, as in Mcintosh and other red apples. 

 Arthur ^ made a critical study of the effect of light of various wave lengths, 

 of temperature, and other factors on the development of the red pigment 

 in the Mcintosh apple and other fruits. This work did much to clarify 

 previous disagreements in the literature on this subject. Fig. 132 shows 

 some of the more important findings. A of this figure shows that short 

 ultraviolet rays of a mercury vapor lamp in quartz kill the apple tissue 

 within 30 minutes and render the tissue incapable of later developing the 

 red pigment. B of this figure shows that the apple is injured by radiation 

 with light that is strong in infrared rays. An apple has a small specific 

 surface when compared with a leaf and it is highly cutinized, which means 

 that it loses heat slowly either by radiation or transpiration. W of this 

 figure shows that the ultraviolet transmitted by window glass has low 

 effectiveness in developing the red pigment. P and C show that the shorter 

 ultraviolet rays transmitted by Corex or Pyrex glass gave good color 

 development within 43 hours in green Mcintosh apples picked on August 25. 

 The author concludes that the highly effective region of the spectrum lies 

 between 312 and 290 mju, or beyond the ultraviolet transmitted by window 

 glass. The visible portion of the spectrum from 600 mn to the ultraviolet 

 will induce pigment development but it is very slow in action. The author 

 finds that 15° C (59° F) is the most effective temperature for pigment 

 development. 



Anthocyanins appear in many plant cells, mainly in early spring and in 

 autumn at times of low temperatures ; under these conditions soluble sugars 

 are also abundant in plant organs. This has led many workers to conclude 

 that high sugar favors anthocyanin development. Arthur ^ finds that low 



