408 BOTANICAL GAZETTE (JUNE 
In root tips fixed after an exposure of one hour to /20,000, 
mitosis was arrested; after 16 hours the root tips were dead. After 
20 hours in 2/30,000 there were no mitoses, and the cells of the 
dermatogen, outer periblem, and meristem were dead and disinte- 
grating. The nuclei of the cells not disorganized were of normal 
size and occasionally contained two nucleoli. 
After exposure for 40 hours to /40,000 there were only rare 
mitoses. The outer cell layers were plasmolyzed, the mid-plerome 
cells were vacuolate, and the persistent nuclei shriveled. The 
nuclei of the larger number of the other cells of the plerome were 
resting and very frequently contained two nucleoli. The apex of 
the tip was dead, but some development in the plerome region was 
evidenced by the thickened walls of the cells destined to form the 
fibrovascular bundle. These thickened cells extended down to within 
2™™ of the end of the radicle. At 3™™ from the apex of the tip 
there was an area of hypertrophied periblem cells (fig. 1) which had 
developed to several times the normal size, producing distortion of 
the radicle and giving it a swollen edematous appearance on one side. 
Since the nuclei in these cells were disorganized, a corresponding in- 
crease in size could not be determined. 
In tips exposed for 6 hours to 2/50,000 there were a few division 
figures. An exposure of 20 hours to this concentration killed all the 
cells in the meristem region. Many of the middle periblem cells 
were greatly enlarged, and in the others practically all nuclei were 
in the resting stage and were rich in chromatin. Cells with two 
nucleoli were very common in the plerome region, where also a few 
cells were observed containing three nucleoli. In general appearance 
the plerome cells resembled those shown in jig. 2b. After 44 hours 
practically all the cells were dead and disintegrating. 
An exposure of 20 hours to »/70,000 killed the outer layers of 
cells, but in the inner periblem normal chromatic figures were present, 
and as in the preceding cases the achromatic figures were obscure. 
At the end of 46 hours large vacuoles appeared in the cytoplasm of 
many cells, and frequently so crowded upon the nucleus that it was 
driven to one side of the cell. Nearly all of these cells were enlarged 
in size and irregular in outline. In the inner periblem practically 
all stages of the chromatic figure occurred, although very few cells 
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