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In most other respects a tendril acts as if it were one of several revolving internodes, which all move together by successively bending to each point of the compass. There is, however, in many cases this unimportant difference, that the curving tendril is separated from the curving internode by a rigid petiole.
Hence, perhaps, it is that the petioles have acquired a high degree of sensitiveness as a compensation for the little power of movement in the shoots. The petioles are bowed downwards, and have the same general hook-like form as in C. viticella. The medial petiole and the lateral sub- petioles are sensitive, especially the much bent terminal portion.
I took a twig, thinner than the petiole itself, and with it lightly rubbed several petioles four times up and down; these in 1 hr. 45 m. became slightly curled; the curvature increased during some hours and then began to decrease, but after 25 hrs. from the time of rubbing a vestige of the curvature remained.
All the other species of Vicia, I believe, bear tendrils; but the bean is stiff enough to support its own stem, and in this species, at the end of the petiole, where, according to analogy, a tendril ought to have existed, a small pointed filament projects, about a third of an inch in length, and which is probably the rudiment of a tendril.
This hook, the lateral petioles being directed a little upwards; forms an excellent grappling apparatus, by which the leaves readily become entangled with surrounding objects. If they catch nothing, the whole petiole ultimately grows straight. The main petiole, the sub-petioles, and the three branches into which each basi-lateral sub-petiole is generally subdivided, are all sensitive.
If we inquire how a petiole, a branch or flower-peduncle first became sensitive to a touch, and acquired the power of bending towards the touched side, we get no certain answer. Nevertheless an observation by Hofmeister well deserves attention, namely, that the shoots and leaves of all plants, whilst young, move after being shaken.
A stick placed near a revolving stem was soon struck by the petioles which stand out at right angles, and the revolving movement was thus arrested. The petioles then began, being excited by the contact, to slowly wind round the stick. When the stick was thin, a petiole sometimes wound twice round it. The opposite leaf was in no way affected.
In C. montana it is confined to the main petiole, and has not spread to the sub-petioles of the three leaflets; so it is with young plants of C. calycina, but in older plants it spreads to the three sub-petioles. In C. viticella the sensitiveness has spread to the petioles of the seven leaflets, and to the subdivisions of the basi-lateral sub- petioles.
The leaf is removed but a small part of the stalk or petiole is retained with the bud to serve as a handle. A boat-shaped or shield-shaped piece of bark is removed with the bud. This piece, known technically as a "bud," is inserted in an incision on the stock, so that it slips underneath the bark and next the wood, with only the bud itself showing in the slit; it is then tied in place.
On passing the Shire we observed great quantities of the plant Alfacinya, already mentioned, floating down into the Zambesi. It is probably the 'Pistia stratiotes', a gigantic "duck-weed". It was mixed with quantities of another aquatic plant, which the Barotse named "Njefu", containing in the petiole of the leaf a pleasant-tasted nut.
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