ULTRA-VIOLET AND SEED PLANTS 875 



destructive action of the short wave-lengths present in solar radiation. 

 Kohler (46) using the cadmium line at 2750 A showed that cuticularized, 

 suberizcd, and lignified walls were not penetrated by waves of this length 

 and explained the greater resistance of some leaves to ultra-violet on the 

 basis of differences in the degree of penetration of the harmful rays. 

 Schulze (91) by photomicrographs also demonstrated that the cuticle, 

 epidermis, and xylem absorb strongly in the region of 2800 A, while 

 parenchyma, phloem, and young cambium are quite transparent to this 

 region. He found strong absorption to occur in the middle lamella. 

 He and Kohler showed that the nucleus absorbs strongly at wave-lengths 

 of 2800 and 2750 A, respectively. Dhere and De Rogowski (23) found 

 that pure chlorophyll was remarkably transparent to ultra-violet, but 

 that the natural chlorophylls in ether solution had a common absorption 

 band near the middle of the ultra-violet spectrum, which would be at 

 about 3040 A. 



More recently Bucholtz (8), Gola (32), Metzner (60), and ShuU and 

 Lemon (103) have published their observations on this subject. Bucholtz 

 noted that leaves of Mnium and the stamen hairs of Tradescantia refieza 

 were much more resistant to the lethal action of ultra-violet than bacteria 

 and paramoecia, probably because of the greater opaqueness of the cells 



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of higher plants. He used wave-lengths of the range 3654 to 2378 A. 

 Gola's studies (32) were on the reflection and absorption of the region 



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3400 to 3800 A by the flowers, leaves, and fruits of numerous plants. 

 His results were determined by impressions made on photographic plates 

 by the plants when photographed by means of radiation in this region of 

 the spectrum. Flowers and leaves containing flavone groups were found 

 to absorb this radiation. Flowers containing only carotinoid pigments 

 reflected it. Waxy coverings on leaves and fruits made heavy impres- 

 sions on the photographic plates. The reflecting capacities of the outer 

 coverings of plants and the absorbing capacities of the flavone groups 

 were looked upon as defense mechanisms of the plant against this type 

 of radiation. 



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Metzner (60) also using long-wave ultra-violet (3500 to 4000 A) 

 determined photomicrographically the penetration of this region into 

 various plant parts. The older work of this type was carried out with 

 radiation of shorter wave-lengths not present in sunlight. His photo- 

 graphs showed that cellulose, hemicellulose, and silicified walls were 

 relatively transparent to this region. The plasma and nucleus absorbed 

 weakly. On the other hand, corky and cutinized walls, and lignified 

 ones to a lesser extent, prevented penetration by this region. Particu- 

 larly strong absorption occurred in the cell sap of the epidermis, guard 

 cells, and mesophyll of many plants; this absorption was thought to be 

 due to the presence of tannins and flavones and to be of biological 

 significance. 



