834 BIOLOGICAL EFFECTS OF RADIATION 



normally found in the air chambers, in the blue light, and in the green 

 light very few of the air chambers were formed. Chloroplasts were 

 disk-shaped under the red filter and round under the blue. 



Thus we find that there are some apparent contradictions in the 

 results of the various investigators, especially with reference to the 

 photomorphic effect of the blue light. Of course, in Forster's (10) 

 work the gemmae used in the experiments are not supplied with a great 

 amount of reserve food. It might appear then from his work, at least, 

 that the red light is there playing a photomorphic role, since in the blue 

 light the air chambers of the thallus were lacking, while in the red these 

 chambers developed. The origin of the assimilatory cells certainly 

 involves more than cell elongation, it requires several cell divisions which 

 were not carried out under blue light. Thus it seems wise in the present 

 state of our knowledge to draw only provisional conclusions concerning 

 the specific effects of light rays of different parts of the spectrum. 



Likewise, we should be cautious in drawing conclusions concerning 

 the effects of light on internal structure. Kohl (16) obtained very 

 marked differences in the internal structure of leaves and of stems of a 

 number of seed plants after exposure to conditions of high transpiration 

 as contrasted with low transpiration. On the other hand, Burgerstein 

 (3, 4), whose monographs summarize and discuss the work of many 

 investigators on transpiration, including effects of various colored lights 

 on this process, makes it clear that the transpiration rate is not the same 

 when plants are grown under lights of different colors. 



Toward all experiments involving either intensity or quality a critical 

 attitude should be maintained, especially in large-scale plant study, 

 in regard to the specific influence ascribed to this factor unless it is quite 

 clear (a) that change in intensity does not at the same time involve change 

 in quality, or the reverse, and (b) that the screens or filters selected shall 

 transmit with sufficient purity the spectral region under study. 



EFFECTS OF ULTRA-VIOLET RADIATION 



Investigators reporting on the effects of ultra-violet radiation are 

 more nearly in agreement with respect to the general effects of this form 

 of radiation than are those reporting on intensity and quality effects. 

 Nearly all investigators of recent years have employed mcrcury-vapor- 

 quartz tubes. There is unanimous testimony concerning the develop- 

 ment of a shiny leaf surface, due to the killing effect on the epidermal 

 cells. Investigators are further agreed that the ultra-violet rays do not 

 penetrate very deeply into plant tissues, that their highly destructive 

 effect is usually confined to one or more superficial layers of cells. 



Siemans (33), Deherain (7), and Bailey (1) recorded damaging effects 

 of a naked electric arc on growing plants but the plants were found to be 

 protected from the damage by filtering the rays through glass. Bailey 



