850 BIOLOGICAL EFFECTS OF RADIATION 



ciency of infra-red plus light as compared with light alone (using the water 

 filters) ranged from 85.5 to 99.0 for pine seedhngs. The temperature 

 in the series of infra-red experiments ranged from 27.0° to 33.5°C. This 

 is presumably air temperature under the bell jars. While no record of 

 the tissue temperature under the two conditions of illumination is given, 

 there is the possibility that this was somewhat higher under the visible 

 plus infra-red conditions since the energy level used was more than three 

 times as great. This possible increase in temperature might account 

 for the slight decrease in efficiency at the higher energy level. Burns 

 noted that the temperature coefficient of photosynthesis at 28°C. is so 

 near one that no effect of temperature can be detected in his experiments. 

 Lundegardh (17), however, working with broad-leaf plants, has shown 

 that this quotient is often less than one with low light intensity and higher 

 carbon dioxide concentrations (1.22 per cent) and that the amount of 

 carbon dioxide assimilated falls off with increasing temperature from 25° 

 to 35°C. Other factors such as respiration and stomatal opening should 

 also be considered. In the absence of further data it cannot be regarded 

 as definitely established that the infra-red of wave-length longer than 



o 



11,000 A is detrimental to photosynthesis. 



Attention has already been directed to the injuries on leaves produced 

 by high infra-red along with high temperature and high relative humidity. 

 It is probable that only under extreme conditions could such an injury 

 result except where plants were limited in the amount of water supplied 

 them. There is the possibility of injury, however, in the case of plant 

 organs exposed to infra-red, which, on account of the nature of the epi- 

 dermis, either cannot lose moisture or lose it only in insufficient amounts 

 to take care of the radiation supplied. Arthur (1) has observed a case 

 of this kind. Apples were kept in an insulated box, the air temperature 

 of which was maintained at about 2°C. A 500-watt Mazda lamp was 

 suspended over the box at a distance of approximately 30 in. above the 

 fruit. A filter made up of Coming's heat-transmitting glass was placed 

 over the box at a distance of 24 in. below the lamp. After five days' 

 exposure to the infra-red output of the lamp, a wrinkled, necrotic area 

 developed on the side of the apples exposed. The internal temperature 

 of the apples was found to be about 20°C. higher than the surrounding air. 

 While this temperature was considerably above that of the air, it was 

 believed that it was not sufficiently high to have caused the injury on the 

 upper surfaces of the apples. The injury was thought to have been 

 produced by the direct action of infra-red on a tissue which absorbs it 

 freely. A second test was made using the visible output of the same 

 lamp as transmitted by a filter consisting of 1 cm. of water and Coming's 

 Aklo heat-absorbing glass. No injury was produced. It should be 

 pointed out, however, that the energy value in this case was much less 

 as the distance from the lamp to the fmit remained the same. It is 



