1905] MCCALLUM—REGENERATION IN PLANTS 117 
These experiments all show, at least in the bean, that an increase 
of water at any point along the stem will not in itself incite the forma- 
tion of roots, and that root primordia will be organized when the cells 
there contain much less water than when growing normally. Their 
subsequent development depends on sufficient external moisture to 
prevent them from wilting. 
In Salix, root primordia 
are laid down early along the 
stem in the vicinity of the 
buds. In some species, at 
least, contact with water, as 
KtEBs has shown, will incite 
these to active growth. It 
does not necessarily follow, 
however, that this is due to 
'the increased absorption of 
A 
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Fic. 11 Fic.. 12 
water by these cells, as KiEBs maintains. The following experi- 
ments throw some light on this. 
Experiment 33.—Three pieces of Salix glaucophylla stem two 
years old were cut off from plants which were growing in pots in the 
greenhouse and weighed. The aggregate weight was 27.5". These 
were placed horizontally in a chamber where the air was just moist. 
