18 
GENETICS: SINNOTT AND BLAKESLEE Proc. N. A. S. 
fork of the stem; (3) the width of the angles formed by the branches at 
the two second forks; (4) the width of the angles formed by the branches 
at the four third forks; (5) the length of the internodes forming the first 
fork; (6) the lengths of the two pairs of internodes forming the two second 
forks; (7) the lengths of ten of the younger internodes (all of the same 
age) near the ends of the branches; (8) the diameter of these ten inter- 
nodes; and (9) the ratio between the length of the two branches above 
each of these ten internodes. In each type studied, a considerable number 
of plants were measured and the measurements averaged. 
The various factor mutations were compared with the normal type 
(purple-flowered, spiny-fruited and many-noded). The white-flowered 
mutant was not found to differ appreciably from the normal type. The 
smooth-fruited form, however, has significantly narrower angles, shorter 
internodes, greater stem diameter and greater difference between the two 
branches at a fork than does the spiny-fruited one; and the few-noded 
type has a significantly shorter trunk, shorter internodes, narrower stem 
diameter and greater difference between the two branches at a fork than 
does the many-noded form. These constitute excellent examples of the 
manifold effects of a single factor upon the plant body as a whole. 
Internally, few constant and characteristic differences between these 
mutant types and the normal form have as yet been distinguished, but 
in view of the notable differences in external structure it seems likely 
that a more complete and detailed anatomical study may reveal such 
differences. 
An investigation was made of the gross external structure of several of 
the chromosome mutants with a single extra chromosome and each was 
found to display characteristic and significant differences from the normal 
type. Compared with the normal, "Reduced," for instance, shows a 
significant decrease in trunk length, angle width and stem diameter; 
"Cockleburr" an increase in angle width and a decrease in stem diameter 
and in internode length; "Globe" a decrease in internode length and an 
increase in stem diameter and in the difference in length between the 
two branches at a fork, and "Buckling" an increase in angle width. 
Furthermore, a number of these chromosome mutants differ radically 
from the normal type in their internal structure. Both "Poinsettia" 
and "Wiry," for example, tend to have an asymmetrical central cylinder, 
and opposite the thin portions of the cylinder the cortex breaks open and 
produces a characteristic lesion. In these mutants wedges of wood also 
extend for some distance into the pith. In both "Cockleburr" and 
"Wedge," vessels are much more abundant than in the normal type and 
show a great variation in size. "Reduced" has a well -developed ring of 
sclerenchyma at the outer edge of the pith and the inner edge of the cortex. 
"Globe" tends to have thick-walled vessels, very round in cross section. 
