X-RAY EFFECTS ON GHEEN PLANTS 973 



which has been a subject of controversy for 35 years, still remains to be 

 solved. Some investigators believe that they have shown stimulative 

 effects of light doses, while others have referred only to a transitory 

 acceleration of development. The majority of the later workers, how- 

 ever, have found that with carefully controlled experiments carried on 

 to maturity, with records taken at the end of the life cycle, doses regarded 

 as stimulative do not regularly cause growth increment in vegetative 

 parts. With comparable units of measurement available in the use of 

 the r-units, within a few years there will undoubtedly be agreement con- 

 cerning the effects of light doses. The experimental evidence to date 

 seems to indicate that stems are less sensitive to the action of the rays 

 than roots; lateral roots are much more susceptible than the main ones. 

 Dry seeds are particularly resistant to the rays; even extremely heavy 

 doses do not prevent the cotyledons from appearing. The effects are 

 materially influenced by the stage of development; seedlings are much 

 more sensitive than dry seeds. Respiration of rayed seedlings parallels 

 growth, that is, if growth is retarded, respiration is likewise. Nyctinastic 

 and seismonastic movements are reported to be inhibited by rays of 

 sufficient intensity. 



MORPHOLOGICAL AND HISTOLOGICAL EFFECTS OF X-RADIATION 

 MORPHOLOGICAL EFFECTS ON LEAVES, AERIAL AND UNDERGROUND STEMS 



Morphological and histological effects accompanying X-raying have 

 been almost universally noted in leaves, stems, and flowers by those who 

 have grown plants to maturity. The destructive action of medium and 

 heavy doses is marked in many species. X-rays seem to kill certain 

 cells and leave others free to proliferate. Deformed organs and unusual 

 branching then result. 



Johnson (19, 20a, 22, 22a) found almost universal production of leaf 

 anomalies unless the species irradiated was extremely ray-resistant. 

 Unfolding leaves from seeds or seedlings irradiated with medium or heavy 

 doses presented a peculiar pebbly appearance soon after treatment. As 

 the leaves grew older, light-green areas intermingled with normal green 

 gave a mosaic or variegated aspect. The early leaves seldom show^ed 

 complete recovery, but those produced later by these same plants 

 appeared normal in all respects. Anomalies in shape were frequent. 

 Simple leaves were often notched at the apex, deeply forked, or occasion- 

 ally split into two independent leaves, attached at the same point on the 

 main stem. Many of the leaves in the growing tip showed incurling and 

 ruffling of the margins. In a compound leaf such as the tomato, leaflets 

 were often twisted for one-half to one-third of their length ; in many cases, 

 for half the length of the leaflet, the blade on one side was absent. Fusion 

 of leaf parts was common and occasionally the widened base of the leaflet 



