12 PENHALLOW ON MECHANISM 
of growth; and the conviction becomes more firmly impressed upon us that this position 
is not only fixed, but that it coincides with the vibrogen bands. 
According to this view, all movement would be primarily due to these three bands, 
supplemented by less vigorous growth in the intermediate tissues. Therefore, all depar- 
tures of motion would arise primarily from the two vibrogen bands traversing the sides 
of the tendril, and all latitudes of motion would be due to that vibrogen traversing the 
upper side of the tendril arm. Any deviation from strictly lateral or vertical oscillations 
must then arise as resultants of activity, either between two vibrogen bands, or between 
one vibrogen band and intermediate tissue of slower growth. Finally, the torsion already 
shown is to be regarded as having its origin in, and as compensating excessive growth in, 
one or all of the vibrogen bands of tissue. 
SPASMODIC MOVEMENT.—It has been noted that towards the end of the circumnuta- 
tions, periods of rest alternate with periods of activity; that the whole action lacks vigor, 
and that there is a failure to accomplish those grand sweeps which are so conspicuous in 
the earlier period of activity. 
These features are undoubtedly to be referred to gradually increasing lignification in 
the wood and bast cells, and the modified conditions of tension which necessarily result 
from this. As the bast cells, particularly, increase in thickness, their degree of resistance 
or of positive tension correspondingly increases, while at the same time the growth of the 
parenchyma tissues continues at a nearly uniform rate. So long as the bast remains thin- 
walled and capable of its maximum extension or response to conditions of external 
tension, for such period is the normal relation between it and the more actively growing 
tissue preserved, and this is marked by regularity and rapidity of motion in the whole 
organ. With excessive disturbance of the normal relations, the equilibrium is disturbed 
in the direction of the more resisting structure, and this finds expression, first of all in 
slow and spasmodic movement, and finally in the completion of the spiral, which is 
always developed freely, without contact, at the end of the period of cireumnutation. 
CoILING ABOUT A SupporT.—Coiling about an object with which the tendril comes in 
contact, has already been discussed indirectly, though it may be well to refer to one or 
two facts more particularly. The coiling of the tendril tip about the point of contact, is 
the direct result of irritation, as both Sachs and Darwin have already shown, and as the 
latter’ explains, it is developed by a shortening of the side in contact with the object, the 
same change, 1.e., condensation of structure and release of tension, operating here as in 
previous cases; and with Darwin, we can hardly agree with Sachs,* that the coiling is in 
any way due to accelerated growth in the unirritated side. 
When once growth in length is arrested, as it appears to be soon after coiling is 
effected, the rapid hardening of all the parts appears to be the prevailing change. In this, 
however, it is difficult to conceive that the mechanical irritation has produced more than 
a very limited effect in advancing maturity. On the other hand, it rather appears that 
each tendril arm has a normal period of growth, which is completed only when the wood 
and bast cells have reached their full degree of maturity. If at the end of this period the 

! Climbing Plants, 181. ? Text-book, 869. 
