210 STUDIES IN PLANT REGENERATION 
the increased amount of tissue between the bundles in the flat part. 
As the plant grows older, these flattened parts again become cylin- 
drical. In this case the subsequent rounding-off is caused by the 
interpolation of new bundles between the old, so that eventually a 
continuous ring is formed, which thereafter widens from the cam- 
bium in the usual manner. The difference between this growth 
and that shown in the specimen of Fig. 7 is at once apparent. In 
the normal thickening, the whole stem changes its form, and the 
new bundles are formed at an equal rate in all parts. In the re- 
generated specimens, however, only about a third of the stem took 
part in the change, while the rest remained flat and unthickened as 
before. Even in the rounded part the new bundles had formed 
unevenly, so that every little elevation was visible on the reverse 
side from that shown in the figure. The unthickened edges would 
probably have been split off, in time, and growth would have con- 
tinued regularly. Signs of such a splitting were just visible 
when the plant was removed for preservation. This behavior 
is also to be contrasted with that of the specimens from which 
all buds were cut (Ехр. 14). Though the latter were growing 
for a period very nearly as long as the one just described, no 
secondary thickening took place. Evidently the presence of 
a growing point in some way regulates even a growth in diame- 
ter. 
The stem of Phyllocactus resembles that of Muhlenbeckia in 
being flat and leafless. Іп other respects also, this plant recom- 
mendis itself for the same series of experiments as the latter stem. 
Here only a very narrow triangle of tissue needs to be cut from 
the edge of a node in order to excise the normal bud-producing 
region. The bulk of the stem thus remains intact, and is able to 
continue its work of photosynthesis as before. 
Experiment 18.-- Portions of stems of Phyllocactus (sp.?) were 
subjected to incisions at the nodes similar to the ones described 
for Muhlenbeckia, When placed in an upright position they also 
produced the roots only from the lower cut surface. There is 
in these flat stems a rounder thicker central strand which is 
sometimes referred to as the “midrib.” Most of the roots took 
their origin from the cut end of this strand, though an occasional 
root came from the flattened part. No shoots appeared. Laterally 
