289 STUDIES IN PLANT REGENERATION 
of so-called “ reversal of polarity” have been found. When, for 
example, the production of a callus at the apical end of a poplar 
stem is prevented by growth in dry air, or by a cap of sealing-wax, 
the basal callus produces the shoots; and instances of reversal 
in root-cuttings of Taraxacum have been referred to already. Тһе 
experiments recorded above showed organs in unusual positions in 
the case of the shoots on the parsnip and horseradish and the 
roots on thin slices of sweet-potato. Yet, as Tittmann first em- 
phasized, only one pole is ever affected in these results, while the 
other produces its normal type of organ or nothing.* Апа, further, 
though by unusual conditions the position of the regenerated struc- 
ture has been modified, the piece cannot be said to have altered its 
polarity ; for as soon as it is restored to favorable surroundings, it 
will produce organs in the normal way. That this “ normal мау” 
must have reference to апу peculiarities which the uninjured plant 
manifests has been made clear by some of Goebel’s + later work. 
Klebs,{ while admitting the polar tendency in cuttings, as a re- 
sult of a series of experiments on both uninjured and cut willow 
twigs, came to the conclusion that this disposition could not be the 
determining factor in evoking the appearance of the roots. By 
local application of water he succeeded in inducing the apical end 
of twigs still on the plant to root; and in cuttings, by varying the 
temperature at the two ends or by inversion whereby the basal епа. 
extended into dry air he found that the roots developed only at the 
apex. He accordingly states that the real cause of the appearance 
of the roots lies in the fact that something is supplied which was 
previously present in insufficient quantity for development. In the 
case of roots, at least, this necessary factor is a local abundance of 
water. Roots, he says, normally appear at the basal end of cut- 
tings because here there is a contact with moist earth and an ab- 
` sence of light. Не criticises Vochting’s inversion-experiments on 
the score that in many species of willow the rind is too thick to be 
easily permeable to water ; but if these twigs are partially denuded . 
of bark so as to allow water to enter, an equal number of roots 
appear at apical or basal end independently of polarity. He con- 
*Tittmann, H. / с. 167 seg. 
t Goebel, К. Allgemeine Regenerationsprobléme. 
fEKlebs,G. Willkürliche Entwickelungsinderungen 
Flora 05: 405. 1905 
bei Pflanzen, 96-124. 1903. 
