CONTROLLED ENVIRONMENTAL CONDITIONS 339 



(7) Underground storage organs of most plants develop on a short day. 

 The potato is an exception ; at 68° F (20° C) it tuberizes well on long days 

 or even under continuous illumination. Storage of foods in the root of 

 four-o'clock is also favored by the long day. 



(8) The ratio of assimilable carbohydrates to total nitrogen in plants was 

 varied greatly by the several conditions of growth in the constant-light 

 rooms and gantry crane greenhouses. This was especially true in the 

 radish, lettuce, and others, but was far less marked in salvia, small grains, 

 corn, and others in which nitrate absorption is apparently regulated. The 

 day length rather than chemical composition, or C/N ratio, determined 

 the flowering of all photoperiodic plants. Plants indifferent to day length as 

 regards flowering, flowered over a wide range of C/N ratio, as did the 

 photoperiodic plants. 



(9) Cultures in the spectral greenhouses indicate that the complete 

 visible solar spectrum is necessary for perfect development of green plants 

 and that ultraviolet has some, though a minor, importance for the form and 

 composition of plants. Plants grouTi in light from the long end of the visible 

 spectrum alone are spindly mth weak stems and small thin leaves, or are 

 otherwise inferior. Plants grown in light from the short end of the spectrum 

 are sturdy, \vith well-formed deep green leaves, but are low in stature. It 

 is possible that if sufficient intensity of blue-violet were available it would 

 yield plants similar to those produced by the complete spectrum. A bal- 

 anced spectrum seems to be necessary for plants such as we see in nature. 

 There are indications that sodium vapor lamps supplemented by mercury 

 vapor or tungsten lights also give a balanced spectrum. 



(10) Although ultraviolet rays are of minor importance in determining 

 the stature and form of green plants, at least at sea level, they affect plants 

 in several other ways, (a) Rays of the solar spectrum shorter than those 

 transmitted by A\dndow glass, 312 to 290 mn, give very rapid development 

 of the anthocyanin pigment in IMcIntosh apples. This pigment does not 

 develop in darkness and only slowly in long ultraviolet and shorter visible 

 rays. These rays also hasten the development of the pigment in cranberries 

 and other fruits that develop the pigment in darkness. Low temperature, 

 15° C (59° F) is favorable for the anthocyanin production in Mcintosh 

 apples. (6) Short ultraviolet rays, 290 m^u or longer, induce the formation 

 of vitamin D in most green plants, but it is much more economical to irradi- 

 ate the animal directly than to feed irradiated green plants, (c) Ultraviolet 

 irradiation of plants increases phosphorus and calcium absorption by 

 increasing vitamin D in the plants, (d) Shorter ultraviolet rays from mer- 

 cury vapor quartz tube are very injurious to plants, but rays longer than 

 290 m/i do no injury, (e) Ultraviolet irradiation of Digitalis seedlings does 

 not increase the cardiac glycosides in the seedlings or in older plants gro\\Ti 

 from such seedlings. (/) Ultraviolet rays transform inactive fmws-cinnamic 

 acid into ctVcinnamic acid, an active plant hormone. They transform a 

 number of other inactive compounds into active hormones. 



