100 BOTANICAL GAZETTE [AvGust 
as they have throughout the greater part of the plant kingdom, out 
knowledge of what these conditions are is very obscure. We know 
little enough of the external factors concerned and almost nothing 
at all definite about the internal ones. When a part of the plant 
body is removed, many factors are secessarily disturbed. The nut: 
trive conditions may be profoundly altered, 
as also may be the water relation. The 
influence of the wound itself may be impor 
tant, and independent of these the mere 
absence of the organ may in itself be of fat 
reaching influence. How far any of these 
may be responsible for regeneration is nol 
clear. Various theories have been proposed, 
but none have as yet been supported by 
adequate experimental evidence. 
Intimately associated with this problem 
of regeneration is that of polarity, for almost 
invariably the new structures occur in such 
a manner as to exhibit this remarkable 
phenomenon (fig. 2), and if we can deter 
mine the exact cause of the appearance © 
roots or shoots in an isolated piece of Salis 
stem, for example, the reason for thet 
development at certain places only may be 
apparent. At present we are blocked @ 
the outset by not knowing, at least undet 
most circumstances, the stimulus whi 
incites their development at all. a 
In conducting some investigations this 
—_ subject it soon became evident that the bes! 
Fic. 2 method of attack would be to take all the 
possible factors and work on them 5] J 
rately, subjecting each, one at a time, to a more exact physiolog 
analysis. The effort was made to determine whether the pr 
cause in any given case of regeneration is a necessary part of 
| 
| 
i 
: 
A 
| 
| 
‘ j . ; ; f° 
stimulus, by endeavoring to devise conditions under which the rege" 
ation could be induced to occur in its absence. By this proce 
