830 BIOLOGICAL EFFECTS OF RADIATION 



ANATOMICAL EFFECTS OF DARKNESS 



It is well known that an effect of darkness is to produce etiolation. 

 The generalization is also made that the effect of the red rays is very 

 similar to that of darkness. Thus a plant subjected to red light becomes 

 etiolated, though it may develop chlorophyll or retain its chlorophyll 

 and function in carbon assimilation. Anatomical investigations on 

 etiolated organs have been made by several investigators. A series of 

 studies by Priestley and his students is discussed below. Priestley (26) 

 describes the general anatomy of the etiolated and normal plant of 

 Vicia Faha L. and of Pisum sativum L. These plants show a very charac- 

 teristic growth response in darkness. It is typical of the response to be 

 expected of many dicotyledonous plants. In darkness, there is attenua- 

 tion and not only an absolute reduction in the cross-sectional area of 

 the stem of Vicia and Pisum but also a change in proportions of cortex 

 and stele. Along with the decrease in the diameter of the stem, there 

 is an even greater decrease relatively in the stelar area of the cross 

 section, that is, the stele suffers a greater relative reduction than the 

 cortex. In etiolated plants exposed to an hour of dayUght per day, these 

 conditions have been found to be intermediate. 



In a paper by Priestley and Ewing (27) a very striking difference 

 is pointed out in the development of the endodermis. The endodermis 

 may be absent from the stem in a normal plant of Vicia, Pisum, and many 

 other dicotyledonous plants, or, when the plants are grown in full day- 

 light, it may be present only at the very base of the stem just above the 

 cotyledons. In plants subjected to etiolation this layer is more fully 

 developed in the stem, often with the characteristic Casparian rings. 

 However, the authors point out that observations on etiolated speci- 

 mens of Phaseolus sp. indicate that this plant does not respond by 

 producing an endodermis upon etiolation. Furthermore, there are 

 many dicotyledonous plants, for instance, some of the mints, which show 

 an endodermis when normally grown in light. 



After further study of this feature, Priestley (26) pointed out that 

 in the very young tip of the plumular shoot of an etiolated plant no 

 primary endodermis can be found, but that a well-marked starch sheath 

 surrounds the stele as well as the cortical bundles, and as we pass to 

 lower or slightly older levels of the yoimg stem the starch sheath dis- 

 appears and is replaced by the endodermis. Thus the starch sheath 

 disappears and gives way to a primary endodermis acropetally as the 

 stem develops in darkness. The effect of darkness upon the structural 

 growth "has mainly to be looked upon as a change in sequence of the 

 developmental stages by which differentiated tissues are being formed 

 at the growing apex." In darkness the stem tends to take on some of 

 the internal characteristics of roots. 



