HISTOLOGICAL EFFECTS 2 27 



Two facts stand out clearly in comparing C with D. First the 

 smaller size of the medullary and cortical cells in C, and second the 

 greater thickness of the cortex in D. Cambium is present in both, 

 but the xylem cells are much smaller in C, and here, also, histolog- 

 ical differentiation is slightly more advanced, the cell-walls of the 

 xylem being thicker in proportion to the diamater of the cells. These 

 differences in details combine to give C the appearance of a more 

 mature tissue-complex than D. 



If we compare the histology of the epicotyls, A and B (plate 3), 

 of these same two plants, the same kind of differences are apparent. 

 Though of precisely the same age (from time of planting to time of 

 fixing), the tissue-differentiation of A is that of a more mature organ 

 than that of B\ ducts and woody cells are more numerous, and the 

 bast fibers are larger. The cells of the cortex are of about the same 

 size in the two epicotyls. 



The FIGURES A, B, D and B^ of plate 4, show the structure 

 of the leaves of seedlings of Zea Mays grown from grains exposed 

 dry to rays from radium of 1,800,000 activity (10 mg.) for 47 

 hours. The grains had their embryo-side in contact with the sealed 

 glass tube containing the radium salt. These seedlings had almost 

 wholly lost their power of responding to gravity by growing upright. 

 Whether the capacity of perceiving the gravitational stimulus had 

 been lost or not, we have no means of knowing, but a comparison 

 of these sections with that of a normal leaf, shown at C, shows a 

 profound structural modification. In D, for example, there is an 

 almost entire absence of any mechanical tissue. 



Here, doubtless, lies the explanation of the failure to grow upright. 

 Even if the gravitational stimulus had been perceived, response would 

 have been difficult or impossible. 



In E the wider part of the section is through the midrib, the part 

 of the blade to the left being wanting. Both D and B are from the 

 same leaf, D being between the midrib and the leaf-margin. In B 

 the epidermal layers appear to be hypertrophied, while in A there is 

 both hypertrophy of the epidermis and atrophy of the mesophyll 

 cells. In B the spongy parenchyma is greatly hypertrophied. 



Summary 

 In the above examples, where exposure to the rays of radium was 

 followed by retardation of growth, histological examination discloses 



