But the vine clearly gives
us such a case; and it seems to me as striking and curious an
instance of transition as can well be conceived.
Cissus discolor.--The young shoots show no more movement than can be
accounted for by daily variations in the action of the light. The
tendrils, however, revolve with much regularity, following the sun;
and, in the plants observed by me, swept circles of about 5 inches in
diameter. Five circles were completed in the following times:- 4
hrs. 45 m., 4 hrs. 50 m., 4 hrs. 45 m., 4 hrs. 30 m., and 5 hrs. The
same tendril continues to revolve during three or four days. The
tendrils are from 3.5 to 5 inches in length. They are formed of a
long foot-stalk, bearing two short branches, which in old plants
again bifurcate. The two branches are not of quite equal length; and
as with the vine, the longer one has a scale at its base. The
tendril stands vertically upwards; the extremity of the shoot being
bent abruptly downwards, and this position is probably of service to
the plant by allowing the tendril to revolve freely and vertically.
Both branches of the tendril, whilst young, are highly sensitive. A
touch with a pencil, so gentle as only just to move a tendril borne
at the end of a long flexible shoot, sufficed to cause it to become
perceptibly curved in four or five minutes.
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