1905} MCCALLUM—REGENERATION IN PLANTS 113 
reveal other factors than those which have been considered the most 
important. Experiments on this connection are to be mentioned. 
K1EBs (5, p. 109) says that when through wounding or separation 
roots or shoots either infold themselves or are produced entirely anew, 
it is because through this separation just those conditions are brought 
about in the cells which under all other 
circumstances would start into operation 
the same ‘building processes;” and this 
condition in Salix he asserts lies in the 
necessary accumulation of water in the 
parts concerned. Experiments in this con- 
nection are to be mentioned. VOCHTING 
(13), in his classic experiments on Salix, 
obtained roots on pieces of stem when 
placed in moist air, and concluded that the 
cause of root development was the increased 
moisture. This assumes that the stems 
absorbed moisture from the atmosphere, 
and also that if they had not absorbed this 
the roots would not have developed. 
Before we can attribute a result to any 
factor it is necessary to show (1) that that 
factor is always present when the result 
occurs, and (2) that when it is absent the 
result will not occur. Will roots of Salix, for example, develop on 
the stem only after “the necessary accumulation of water;” or can 
we produce them without this, or with even a decrease of water in 
the parts producing them? The following experiments were to deter- 
mine this. For convenience, those on Phaseolus will be described first. 
If the stem of this plant be cut off anywhere and placed in water, 
roots come out abundantly at the lower end (fig. 5). These arise 
from the pericycle entirely anew, no root primordia existing anywhere 
on the stem. 
Experiment 22.—Entire plants, with roots in soil or water cul- 
tures, were placed in the three moist chambers described above. 
‘hey grew rapidly and no roots formed on the stems. There can be 
no doubt that the stems were entirely saturated with water. 
: 
Fic. 5 
