STUDIES IN PLANT REGENERATION 231 
Experiment 55.— Attempts to supply the lack of this nour- 
ishment by growing the pieces in various dilute sugar and peptone 
solutions repeatedly failed. In only one case did a shoot of Com- 
melina grown in a one per cent sugar solution, form a root 4 mm. 
long. In all the other trials, though attempts were made to keep 
down the number of bacteria by frequent change of the solution 
and by the addition of small doses of copper sulphate, the parts 
decayed without giving positive results. It does not seem at all 
unlikely that with better culture methods this regeneration may be 
more often induced. 
The foregoing experiments indicate, therefore, that normally 
the leaf or shoot has at the time of cutting sufficient reserve food 
to initiate the first stages of regeneration. When, however, this food 
is absent and its formation is prevented either by external conditions 
or by the parasitic habit of the part, regeneration is inhibited. 
Given the necessary food supply, the question next arises as 
to what other conditions are responsible for the appearance of 
the organs in regeneration. Vóchting in 1878 gave the answer 
with regard to external influences which all subsequent experiment 
has failed appreciably to alter. In an exhaustive series of investi- 
gations on the effect of light, gravity, water, pressure, and contact, 
Vóchting came to the conclusion that while external conditions 
may have a modifying or an arresting effect on the place of the 
appearance of the organs, these could not be considered as 
primarily the causes of regeneration. Ав was before men- 
tioned he ascribed the practically uniform position of the struc- 
tures formed on root and shoot-cuttings as due to an inherent force 
in the plant which he termed “ polarity." The fact that leaves 
fail to exhibit this polarity, but form both kinds of organs from the 
base, he explained as due to the limited growth of the latter 
organs as contrasted with the unlimited growth of root and shoot. 
His conclusions have not found universal acceptance. Instances 
have been recorded where — as for example in Bryopsis t — the 
root-pole could, by the action of light, be changed into a shoot- 
pole and the reverse. In the higher plants, too, numerous cases 
"E otc. BN. 
f Noll, Е. Über die Umkehrungsversuche mit Bryopsis. Ber. Deut. Bot. Ges. 
18: 444. 1900. 
