XXIV 



THE EFFECTS OF VISIBLE AND ULTRA-VIOLET 

 RADIATION ON THE HISTOLOGY OF PLANT TISSUES 



J. T. BUCHHOLZ 



Department of Botany, University of Illinois 



Anatomical effects of darkness. Effect of different regions of the visible spectrum. 

 Effects of ultra-violet radiation. References. 



Surveying the work which has been done on the effects of the different 

 regions of the visible spectrum on the morphological and anatomical 

 characteristics of seed plants, it appears that investigations, beginning 

 with those of Sachs, have been concerned more particularly with changes 

 in the external form of the plant body. While the effects upon histologi- 

 cal structure have been studied less intensively, important contributions 

 have been made, and it is the purpose of this paper to bring together 

 briefly the available information in this field. In general, the records 

 seem to indicate, as might be expected, that the exposure of plants to 

 various colored lights, representing restricted regions of the spectrum, 

 does not result in many important qualitative histological changes. 

 Under any region of the visible spectrum the stem presents the usual 

 histological regions, that is, epidermis, cortex, and stele (containing 

 xylem, phloem, pith, etc.), but any one of these regions may be quantita- 

 tively affected, the extent of the modification varying considerably. 



It is well known from the work of Stahl (33) and of others 

 that variations in the intensity of illumination with the full spectrum of 

 light will bring about some quantitative anatomical changes in the 

 tissue of the leaves. Stahl (35) demonstrated that in Lactuca scariosa 

 and Iris the leaves have Uttle or no palisade tissue when grown in sub- 

 dued diffuse light, but there is a well-developed palisade layer when 

 grown in bright sunlight. The development of no palisade tissue in 

 weak Ught as contrasted with a high development of this tissue in strong 

 light might even be considered a quahtative change. In the respect 

 mentioned, leaves on the same trees will vary considerably, so that the 

 sun leaves taken from the exposed upper part of the tree will appear 

 different from the shade leaves found in the lower interior of the tree. 



The external growth effects of colored lights are well summarized by 

 Davenport (6). The earlier experiments on the effects of hght intensity 

 are usually included in the textbooks of physiology and anatomy (Jost, 

 13; Haberlandt, 12; Kuster, 19). 



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