EFFECTS OF EXPOSING GERM-CELLS TO RAYS OF RADIUM 243 



photograph showing a later stage of development of a plant whose 

 rosette was asymmetrical. The main stem is developing, but instead 

 of growing vertically up, as normally, it is bent over, growing nearly 

 horizontally. In each instance of this kind the stem bends over 

 toward the smaller side of the rosette, showing that growth in length 

 is more rapid on the other side of the plant. The photograph also 

 shows that on this main stem the leaves on the more rapidly growing 

 side are larger than those on the opposite side. The tip in such 

 plants is usually turned up, apogeotropically, as shown in the figure, 

 and the stem eventually begins to grow vertically. 



As to how the radium rays acted in order to bring about the 

 effects just described, of course we do not know. That the asym- 

 metry was not due to differences of illumination or to crowding is 

 certain from the known conditions under which the plants were 

 grown. Recalling the nutation of a growing stem, caused by the 

 fact that the region of most rapid growth travels around the stem in 

 a direction opposite to that of the nutation, it is possible that the 

 asymmetry of the rosette and the bending of the stem is due to an 

 inhibition of the migration of the region of maximum growth. Thus 

 the effect is analogous to the modification of nutation by a unilateral 

 stimulus in any tropism. 



2. Morphological Asymmetry : Superficially resembling func- 

 tional asymmetry, are results of which figure 71 shows an example. 

 Here the rosette is one-sided, but, on more careful observation, it is 

 seen that the one-sided appearance is due to something more than 

 mere difference in rate of growth. The leaves on one side of the 

 plant are not onl}^ smaller than those on the other, but they are of 

 different shape, being narrower in proportion to length, and with the 

 margin of the basal portion not notched as normally. Furthermore 

 the transition from the character of one side to that of the other is not 

 absolutely abrupt. The individual leaves between the unlike sides 

 are themselves asymmetrical, the side of the leaf next to the broader 

 leaves of the rosette being wider from midrib to margin than the 

 other side. The tips of these bilaterally asymmetrical leaves turn 

 toward the narrower side of the leaf. 



Here, of course, is functional asymmetry, but because the form 

 also is modified and the asymmetry thus accentuated, I have called 

 this result morphological asymmetry . There is a qualitative as well 

 as a quantitative difference. 



