2 26 HISTOLOGICAL EFFECTS 



Here, also, it is seen to be the cambium that is most profoundly 

 affected. It has nearly, though not entirely, disappeared in the ex- 

 posed plant, though some development of xylem and phloem has 

 taken place. The cells of the cortex and pith are also smaller in the 

 exposed plant than in the control. 



Figures C and D, plate 2, represent, respectively, cross-sec- 

 tions of the roots of the same exposed and control plants as those from 

 which A and B were taken. They show the same kind of result fol- 

 lowing exposure to the radium. The cambium is lacking, and the 

 cells of the cortex and pith are much smaller than normally. 



Figures E and E of the same plate (2) show cross-sections of 

 the hypocotyl of the bean (^Phaseohis vulgaris)^ F oi 2. control plant, 

 E oi 2i plant exposed (Exp. 15, p. 98) during imbibition and germi- 

 nation in moist sphagnum, to a Lieber's coated rod of 10,000 activity. 

 The section was made after growth under continuous exposure for 

 five days. 



The most noticeable difference here is in the size of the various 

 tissue elements, especially evident on comparing the cells of the cor- 

 tex and pith. The cambium has practically all disappeared from 

 both the exposed and the control plant, though traces of it are seen 

 in the control, indicating a more tardy tissue-differentiation. The 

 same kind of differences appear in figures E and F of pi-ate 3, 

 which show cross-sections of the stem of two seedlings of Phaseolus 

 vulgaris exposed (Exp. 19, p. loi) as were those of plate 2. In 

 figures E and F (plate 2), growth took place entirely in darkness 

 and in moist sphagnum, no foliage being developed, and no nour- 

 ishment being supplied except tap-water. 



The contrast between figures C and D (plate 3) is most strik- 

 ing. These figures represent cross-sections through the hypoctyls of 

 seedlings of Phaseolus vulgaris of Experiment 11 (p. 95). The seed 

 from which the seedling of C was developed was exposed for 24 

 hours while dry, to the rays from a radium-coated rod, the rod being 

 in contact with the seed. The activit}' of the rod was 10,000. At the 

 close of the exposure the seeds were planted in the soil, and the seed- 

 lings developed under the most favorable conditions of heat, light, 

 moisture, and nutrition from the soil. The section is from the hypo- 

 cotyl twelve days after planting. The average length of the hypocotyl 

 above the surface of the soil was, for the plants from exposed seeds 

 39.60 mm., for the control seedlings 59.25 mm. 



