ULTRA-VIOLET AND SEED PLANTS 867 



of ultra-violet, quality and intensity of visible and infra-red, presence of 

 ozone, etc. Consequently his statistical analyses are not significant as 

 indicating ultra-violet effects. Since he made no attempt to equalize 

 these other conditions or to vary only the ultra-violet in his test plants 

 as compared with the controls, he is hardly justified in assuming that 

 any differences occurring in the test plants can be attributed wholly to 

 ultra-violet. Furthermore, the differences between test plants and con- 

 trols were often very slight and not always consistent, nor was there 

 sufficient repetition of the experiments to justify any very definite con- 

 clusions being safely drawn. A more detailed discussion of Fuller's 

 work is given by Popp and Brown (74, pages 179-183). 



In a recent paper by Stewart and Arthur (107) it is claimed that 

 ultra-violet radiation between 2900 and 3130 A under certain conditions 

 increases the calcium and phosphorus content of certain plants. Higher 

 total Ught intensity may bring about the same result. It is believed 

 that the radiation exerts its influence indirectly by the activation of 

 ergosterol in the tissues of the plant. 



Popp and Brown in addition to previously reported experiments have 

 carried out five series with buckwheat. In two of these, 2-min. daily 

 exposures to radiations from a mercury arc were given for 10 days; in 

 the other two, 3i-hr. daily irradiations were given. In one 2-min. series 

 and one M-hr. series the seeds were planted in soil and the first irradiation 

 given when the seedlings were several inches high. In the other two 

 series the seeds were placed on moist filter paper and cotton and irradiated 

 for the first time 3 days later, that is, after the seeds were well germinated. 

 Fifty seeds per culture were used. Exposures were given to the open 

 arc, to the arc screened by Corex glass, by window glass, by Noviol 

 "0" glass, and by G586A glass. In addition one culture was kept in 

 diffused light with no irradiation, and one was exposed to the ozone 

 only of the lamp. No favorable effects of the ultra-violet used in these 

 experiments were obtained. The injurious effect of the unscreened arc 

 was again manifested. 



INVESTIGATIONS DEALING WITH PLANTS GROWN EXCLUSIVELY UNDER 



ARTIFICIAL ILLUMINATION 



Withrow and Benedict's experiment (123) was unique in that they 

 grew tomato and Coleus plants from seed for a period of three months 

 under artificial illumination as transmitted by various cellophane filters. 

 Theirs was an attempt to determine whether the region 2900 to 3130 A— 

 that region said to be of such importance to animal life — was essential 

 for optimum conditions of plant growth. They state that others have 

 failed to obtain stimulation either because they did not have present a 



o 



sufficient intensity of the rays 2900 to 3130 A, if daylight was the source 

 of radiation, or that in addition to a sufficient intensity of the rays 2900 to 



