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the same manner as it, in common with many other plants, is 
stimulated by contact with moist earth to put out true roots. That 
very many plants possess an irritability which responds to the stim- 
ulation of contact is a well-known fact. Those which respond by 
movements perceptible to the eye may be passed with a mere 
mention, while we consider some of those more directly bearing 
upon the question which we are discussing. Tendril bearers show 
this irritability in a marked degree, as also do the petioles of leaf 
‘climbers. So necessary is this stimulation to the tendrils of Ampe- 
lopsis quinquetolia that they wither and die without it ; upon grasp- 
ing an object, however, they increase in strength, become woody, 
and retain their hold for years. 
Darwin, in his work on “ Climbing Plants,” details a great num- 
ber of curious and interesting experiments which amply demonstrate 
this irritability and its effects. In speaking of the genus Clematis, 
he says, “‘ When a petiole has clasped a twig it undergoes some re- 
markable changes. * * * The clasped petiole in the course of 
two or three days swells greatly, and ultimately becomes nearly - 
twice as thick as the opposite one which has clasped nothing. When 
thin transverse slices of the two are placed under the microscope 
their difference is conspicuous ; the side of the petiole which has 
been in contact with the support is formed of colorless cells with 
_ their longer axis directed from the centre, and these are very much 
larger than corresponding cells in the opposite or unchanged 
petiole.” The sensitiveness of the petioles in this genus, however 
was limited to the younger leaves. Solanum jasminoides gave 
more decided results. Here he found the petiole of the full grown 
leaf still sensitive, and, when it had clasped a support, the subsequent 
changes were still greater. After detailing his examination of this 
plant, he says, “It is a singular morphological fact that the petiole 
should thus acquire a structure almost identically the same with 
that of the axis ; and it is a still more singular physiological fact that 
so great a change should have been induced by the mere act of 
clasping a support.” Many other examples might be presented, but 
it is unnecessary. The irritability which I claim for the stem of 
Rhus toxicodendron differs only in degree from the instances just 
related. This difference might justly be expected when the ends to 
be attained are considered. In the one case, a short lived leaf 
petiole is made the sensitive organ, and serves its purpose for the 
time required; in the other, the whole stem is made sensitive in 
order to support itself for years. It may be urged against this 
theory that unsupported stems do put forth occasional rootlets. 
This objection should not be considered valid. The nature of the 
plant is of course strongly bent toward the production of rootlets, 
and with this evidently strong predisposition, it need not be con- 
sidered strange if some are produced without the stimulation which 
is usually requisite. Some plants put forth adventitious roots from 
the lower portions of their stems which tend, downward and enter 
the earth, but no one would, on this account, claim that as the 
normal type of root growth. The rootlets being put forth in 
response to the stimulation of contact, I consider that eccentricity 
