1905] Mc CALLUM—REGENERATION IN PLANTS 255 
at the two places cannot be due to any difference in the amount of 
water or nutritive material available at the two places, for both may 
be saturated with water and equally well provided with food. Nor 
can it be that the cells just below the cut have any less capacity for 
root production than those just above; for if the cut be made a little 
lower down, so that the cells 
which formed the upper part 
of the lower piece now form 
the lower part of the upper 
piece, they promptly produce 
roots. It is evident that there 
is some factor operating on the 
cells at the one point that is 
hot present at the other; and 
it seems equally evident that - 
this is not a condition of mois- 
ture, of nutrition, or of a wound 
influence, for all of these are 
equal in each case. - As stated 
above, when no cut is made Fic. 9.—Phaseolus: Inverted stem with 
there is no tendency for any of first pair of leaves and adjacent parts of 
these cells to form roots: but internodes. Apex in water, base in moist air. 
: Roots arising only on base. 
4S soon as an incision is made, 
there is a change in the behavior of the cells above it, while those 
below remain uninfluenced. In the present state of our knowledge 
there seem only two possible lines of explanation: (1) as a result of 
the incision, new conditions are added to the cells above the cut 
Which serve as a stimulus inciting them to root production, conditions 
hot acting on the cells just below the cut; or (2) the cells above the 
See have been relieved of some influence that previously prevented 
their growth, an influence still acting on the cells below the incision, 
and which acts on all the cells when no incision is made. 
Recurring to the first idea for a moment, we have noticed that 
there are two different conceptions current. The one is that these 
new conditions added are the results of changes induced by such 
actors as nutrition, moisture, wounding, light, gravity, aeration, 
te. While light retards the development of roots in this case, It 
