INDUCTION, DEVELOPMENT, AND HERITABILITY OF FASCIATIONS. 15 
continues to the end of the life of the plant. If there were fusion of a 
definite number of growing regions there would seem to be a definite limit 
to the increase in the size of the stem and the number of the leaves. Con- 
cresence may, and frequently does, occur as a consequence of the bifurca- 
tion which is so intimately associated with it, but is an accidental rather 
than an essential factor, and succeeds rather than precedes the division of 
the axis. As to the reason for this curious alteration of form in these fas- 
ciated stems, one can speak only theoretically. It may be that the banded 
fasciations arise from lateral injuries in which the inhibition causes the 
meristem to stretch from the point of attack. This might seem to be illus- 
trated in the case of the rosettes of one-sided development and of the injured 
stems plate v, figs. 9 and 16. In the ring-fasciations the injury may be to 
the tip of the growing meristem, and the stresses thereafter distributed in 
a circular fashion. 
Bifurcations are often caused mechanically by the stresses of old and 
broad fasciations, where the unequal growth and the consequent torsions 
strain the large growing region into segments through virtue of its unwieldy 
size. It is to be expected that a fasciation such as that in plate 1 would 
soon divide in this way if allowed to grow to maturity. The splitting of 
the axes may be more frequently mechanical than superficially appears to 
be the case. We must suppose that in its early stages it is often due to 
the stresses of vigorous growth in an abnormally large tip. The tensions 
which are parallel with the vegetative line are greater than those which 
cross it. A slight disturbance of external conditions, and so of the growth, 
upsets the equilibrium, and the tensionis broken. It must be remembered, 
as Nestler has shown (7), that the apex is not level, but undulate, and it 
may be supposed to be constantly changing. Certain delicate adjustments 
of the stresses may keep the equilibrium until alteration in the rate of 
growth, due to inequalities in nutrition over so extended an area, upset the 
balance and free a portion of the axis. In the smaller segments the stresses 
are not so great; consequently there is increasing tendency toward normal 
growth, and the smallest bifurcations usually in the end completely reverts 
to it. Bifurcations sometimes, both in ring-shaped and in flat fasciations, 
are caused by injury, but in many cases such an origin can not be assigned 
to them. 
Though the development of fasciation has often been referred to external 
stimuli, there is but one direct reference to its connection with insects. 
Molliard (15) in 1900 found the larva of a coleoptera within the fasciated 
stems of Raphanus raphanistrum and of Picris hieractoides, just below the 
banded portions of the axis. He suggested that the parasite modified the 
structure of the vegetative point and changed the mass of the initial mer- 
istem from axial symmetry to the symmetry of a line. 
Peyritsch (5), in his interesting experiments on the production of abnor- 
malities through inoculation with Phytopus, enumerates, among the aberrant 
