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- 

 crcsence 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 i 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 tension is 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 Raphamis raphanistruni and of Picris hieracioides , 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 Phytopiis, enumerates, among the aberrant 



