CONCLUDING BEMABKS. Ill 



parts acting as leaves or as flower-peduucles caa have wholly 



changed their function, and have come to serve as prehensile 

 organs. 



In the whole group of leaf-climbers abundant evidence has been 

 given that an organ, still subserving its proper function as a leaf, 

 may become sensitive to a touch, and thus grasp an adjoining ob- 

 ject. In several leaf-climbers true leaves spontaneously revolve ; 

 and their petioles, after clasping a support, grow thicker and 

 stronger. We thus see that true leaves may acquire all the lead- 

 ing and characteristic qualities of tendrils, namely, sensitiveness, 

 spontaneous movement, and subsequent thickening and indura- 

 tion. If their blades or laminse were to abort, they would form 

 true tendrils. And of this process of abortion we have seen every 

 stage ; for in an ordinary tendril, as in that of the Pea, we can 

 discover no trace of its primordial nature ; in Mutisia clematis^ 

 the tendril, in shape and colour, closely resembles a petiole with 

 the denuded midribs of its leaflets ; and occasionally vestiges of 

 laminae are retained or reappear. Lastly, in four genera in the 

 same family of the Fumariacese we see the whole gradation ; for 

 the terminal leaflets of the leaf-climbing Fmnaria officinalis are not 

 smaller than the other leaflets ; those of the leaf-climbing Adlumia 

 eirrliosa are greatly reduced ; those of the Cori/dalis claviculafa 

 (a plant which may indifferently be called a leaf-climber or tendril- 

 bearer) are either reduced to microscopical dimensions or have 

 their blades quite aborted, so that this plant is in an actual 

 state of transition ; and, finally, in the Dicentra the tendrils are 

 perfectly characterized. Hence, if we were to see at the same 

 time all the progenitors of the Dicentra, we should almost cer- 

 tainly behold a series like that now exhibited by the above-named 

 four genera. In Tropceolum tricolorum we have another kind of 

 passage ; for the leaves which are first formed on the young plant 

 are entirely destitute of laminae, and must be called tendrils, whilst 

 the later-formed leaves have well-developed laminae. In all cases, 

 in the several kinds of leaf-climbers and of tenduil-bearers, the 

 acquirement of sensitiveness by the mid-ribs of the leaves appa- 

 rently stands in the closest relation with the abortion of their 

 laminae or blades. 



On the view here given, leaf-climbers were primordially twiners, 

 and tendril-bearers (of the modified leaf division) were primor- 

 dially leaf-climbers. Hence leaf-climbers are intermediate in 

 nature between twiners and tendril-bearers, and ought to be 

 related to both. This is the case : thus the several leaf-climbing 



