CONTROLLED ENVIRONMENTAL CONDITIONS 323 



exposure. It is evident that rays much shorter than those transmitted 

 through the atmosphere from sunlight injure plants. Purified tobacco virus 

 is inactivated by exposure to the open arc, but the virus within the plant 

 is not inactivated. Irradiation did not stimulate growth or fruit set; the 

 dominant effect is injury by the shorter rays. 



It was claimed that Digitalis seedlings grown under ultraviolet-transmit- 

 ting glass bear much more of the cardiac glycosides than similar seedlings 

 grown under window glass, and that this difference persists even after the 

 plants are gro^\^l for long periods in the open field. Contrary to this con- 

 clusion, Leonard and Arthur -^ find no difference between the glycoside 

 content of plants gro^\^l under Sunlit and '\\nndow glass when all other con- 

 ditions are the same. They mention two other conclusions from the study 

 that are of interest: the rosette stage of Digitalis requires three or four 

 months of exposure to a low temperature (50° F [10° C] or lower) before it 

 will produce a flower stalk, and the concentration of the glycosides in 

 plants 4 to 6 months old is about t^^ice that in plants 9 to 17 months old, 

 whether flower stalks are formed on old plants or not. 



Ultraviolet rays change inactive fraws-cinnamic acid ^^ to the isomer cis- 

 cinnamic, which is an active plant hormone. This change occurs whether 

 the acid is irradiated before or after application to plants. Other phenyl 

 compounds, p-, m-, and o-nitrocinnamic acids, are similarly activated, as 

 are /3-naphthoylacetonitrile and tryptophane. 



Minimum Light Intensity for the Survival of Green Plants 



Green plants make two uses of the organic carbon materials they synthe- 

 size in sunlight; they oxidize them as a source of energy for growth or repair 

 and they build them into new tissue leading to an increase in dry weight. 

 Assuming that all other growth factors are held at a high level or near the 

 optimum, but that the light intensity is gradually reduced, it will finally 

 reach a point where the synthesis during the day is just sufficient to main- 

 tain respiration during the day and night, ^^^th no increase in dry weight, 

 and the plant is able only to maintain its weight, if indeed it does not perish. 

 All the plant materials we use as food or otherwise are possible because the 

 synthesis during the day is in excess of the respiration during day and 

 night. Plants on a forest floor have to grow in a low light intensity as well 

 as modified light quality because some of the light reaching these plants 

 has already passed through green foliage \vith differential absorption of the 

 rays. Of course what is true of plants on a forest floor is true of weeds 

 growing in the shade of crops or crops grouing in the shade of weeds, and 

 in part of plants growing in any sort of shade. 



Shirley ^^' ^^ attacked anew the much investigated problems, What is the 

 minimum daily light intensity in which various plants can persist? and 

 What is the effect of varying light intensity and quality upon rate and 

 nature of plant development? Experiments were run under different inten- 



