May 24, 1924 
Acromania, or “ Crazy-Top ” 
823 
Even if organisms were found to be responsible for the growth disorders, the 
abnormalities would still be ascribed to the secretions or waste products of such 
organisms. Of course, it has long been known that insects could produce galls 
and other localized malformations, but in the growth disorders the whole subse¬ 
quent development of the plant may become abnormal. Instead of a merely 
local irritation, as in a gall or a hexenbesen, the normal course of development 
may be lost suddenly, at any stage in the growth of the plant. (See PI. 8, 9, 
10, and 11.) 
The growth disorders afford additional illustrations of the fact that two distinct 
processes are involved in heredity. The expression or development of the charac¬ 
ters in the individual is distinct from the transmission of the characters. How the 
changes of expression are brought about is still unknown, but it is plain that 
expression can be modified, and in many different ways, by the growth disorders, 
and that the changes may be permanent during the life of the individual organism. 
It has long been recognized that mutilations or defects induced by diseases are 
not inherited, but such accidental injuries are not the same as the changing of the 
course of development by the growth disorders, which continues through the life 
of the plant individual. Except that all of the later growth is abnormal, the 
changes of characters in crazy-top are similar to those that occur in “bud 
mutations,” where single branches show changes of characters that are definite 
and can be propagated as a distinct variety. Also, in such cases the original 
characters are not lost, since reversions to the form of the original variety may 
occur in later bud mutations. 
It has been customary to assume the existence of a* ‘mechanism of heredity'' to 
account for the very definite transmission of characters, but less consideration has 
been given to the idea of a mechanism of expression of the characters, as distinct 
from the mechanism of transmission. Definite ideas of such “mechanisms" are 
still lacking, but it is plain from the growth disorders that the control of develop¬ 
ment may be changed profoundly and in ways that may interfere quite definitely 
with the development of the individual, without corresponding changes of the 
transmitted characters. Though the causes are still unknown, the growth dis¬ 
orders undoubtedly have this remarkable property of interfering and altering the 
adjustment of the mechanism of expression. 
Although the normal hereditary characters are thrown out of expression in the 
affected plants, they reappear in the next generation. The disturbing cause, 
that in some way prevents or masks the expression of the characters, apparently 
is not carried in the seed in most of the growth disorders, though some are difficult 
to distinguish from genetic defects. Yet the abnormal expression of the charac¬ 
ters, as induced by the growth disorders, may go on indefinitely in the vegetative 
or somatic tissues of the plant, in all of the new growth that is formed. Some of 
the mosaic diseases are communicated readily by grafting or by artificial infection, 
as well as by insects or other natural carriers. 
As yet there is nothing to show how the expression of the characters is deranged. 
Even the normal processes of growth are little understood, and we have no con¬ 
ception of what it is that controls the development of the plant and brings the 
normal inherited characters into expression. Though each of the growth dis¬ 
orders shows a general trend of abnormality in one direction or another, the 
greater individual diversity of the abnormal plants would seem to indicate a nega¬ 
tive effect or loss of the normal adjustment of the expression relations of the char¬ 
acters, rather than a positive effect of establishing a new determination of the 
characters, although a certain stability is shown in the expression of the abnormal 
characters. 
