194 CONCLUDING REMARKS. Cuapr. V. 
their terminal leaflets converted into perfect tendrils ; 
in Eceremocarpus I have seen a single lateral branch 
of a tendril replaced by a perfect leaflet; in Vicia 
sativa, on the other hand, leaflets are sometimes 
replaced by tendril-branches; and many other such 
cases could be given. But he who believes in the 
slow modification of species will not be content simply 
to ascertain the homological nature of different kinds 
of tendrils; he will wish to learn, as far as is possible, 
by what actual steps leaves, flower-peduncles, &c., have 
had their functions wholly changed, and have come to 
serve merely as prehensile organs. 
In the whole group of leaf-climbers abundant 
evidence has been given that an organ, still subsery- 
ing the functions of a leaf, may become sensitive to a 
touch, and thus grasp an adjoining object. With 
several leaf-climbers the true leaves spontaneously 
revolve; and their petioles, after clasping a support 
erow thicker and stronger. We thus see that leaves 
may acquire all the leading and characteristic qualities 
of tendrils, namely, sensitiveness, spontaneous move- 
ment, and subsequently increased strength. If their 
blades or laminz were to abort, they would form true 
tendrils. And of this process of abortion we can follow 
every step, until no trace of the original nature of 
the tendril is left. In Mutisia clematis, the tendril, in 
shape and colour, closely resembles the petiole of one 
of the ordinary leaves, together with the midribs of the 
leaflets, but vestiges of the laminz are still occasionally 
retained. In four genera of the Fumariacese we can 
