730 BIOLOGICAL EFFECTS OF RADIATION 



and palisade tissue assumed more normal proportions. Eventually 

 they attained a structure similar to normal plants. 



In anatomical structure etiolated stems show an arrested develop- 

 ment. Priestley and Ewing (78) noted that stems of potato (Solanum 

 tuberosum) and broad bean (Vicia Faba) tended to develop in darkness 

 a structure comparable to that of roots. Dyes injected in etiolated 

 plants could not be forced into the meristematic growing points, while 

 in normal stems the dye readily penetrated the meristem to the surface. 

 They concluded that the failure of the growing apex to unfold and 

 develop normally was due to the lack of nutrition caused by the imperme- 

 ability of the meristem tissue. The meristem is rich in proteins and fats, 

 which probably accounts for its impermeability to nutrient sap. 



Priestley (77), in a later paper, stated that the general anatomy 

 of Vicia and Pisum remained unaltered in darkness except that 

 the third and further internodes did not develop but remained curled up 

 in the plumular hook. The diameter of the stele was much smaller in 

 proportion to the diameter of the stem in etiolated plants. Exposure to 

 light for one hour daily caused a considerable expansion in the stele, but 

 it was still not so large as in a normal plant. In the shoots the stele was 

 bounded first by a starch sheath which was later replaced by a primary 

 endodermis. This starch sheath and primary endodermis appeared to 

 contain fatty substances in the cell wall which rendered them more or 

 less impermeable. It was impossible to plasmolyze the differentiated 

 cortical cells of plants grown completely in the dark, but after brief daily 

 exposures to light these cells plasmolyzed readily. There was consider- 

 able starch in the etiolated shoot which was concentrated just below the 

 meristematic apex. Upon exposure to light this disappeared very 

 rapidly. He concluded that the main morphological structure of etiola- 

 tion is determined by a disturbance of growth at the shoot apex, which is 

 caused by the relative impermeability of the cell walls between vascular 

 strands and those of the meristem. These cells contain protein and fatty 

 substances that form the surface of the protoplast. 



That exposure to light increases the permeability of protoplasm 

 to dyes has been confirmed by many workers. Lepeschkin (58), 

 by shading portions of Elodea leaves with tinfoil, demonstrated that 

 this effect, too, is local in its action, that it increases with intensity up 

 to a certain point (10 per cent of total sunlight or less in Arizona), and 

 that it reacts very quickly to changes in light. Plants taken from the 

 same light conditions and placed for 2 hr. in different light conditions 

 show tremendous differences in dye absorption. 



In discussing the growth of plants in darkness and in light of varying 

 intensities, it is necessary to differentiate between elongation, expansion 

 of foliar organs, and increase in total plant substance (dry weight). 



