Seite - 252 - in The Origin of Species

252 ORIGIN OF SPECIES to a highly complex pollinium, admirably adapted for trans- portal by insects ; nor will he deny that all the gradations in the several species are admirably adapted in relation to the general structure of each flower for its fertilisation by differ- ent insects. In this, and in almost every other case, the en- quiry may be pushed further backwards ; and it may be asked how did the stigma of an ordinary flower become viscid, but as we do not know the full history of any one group of be- ings, it is as useless to ask, as it is hopeless to attempt answering, such questions. We will now turn to climbing plants. These can be ar- ranged in a long series, from those which simply twine round a support, to those which I have called leaf-climbers, and to those provided with tendrils. In these two latter classes the stems have generally, but not always, lost the power of twin- ing, though they retain the power of revolving, which the tendrils likewise possess. The gradations from leaf-climbers to tendril-bearers are wonderfully close, and certain plants may be indifferently placed in either class. But in ascending the series from simple twiners to leaf-climbers, an important quality is added, namely sensitiveness to a touch, by which means the foot-stalks of the leaves or flowers, or these modi- fied and converted into tendrils, are excited to bend round and clasp the touching object. He who will read my memoir on these plants will, I think, admit that all the many grada- tions in function and structure between simple twiners and tendril-bearers are in each case beneficial in a high degree to the species. For instance, it is clearly a great advantage to a twining plant to become a leaf-climber; and it is probable that every twiner which possessed leaves with long foot- stalks would have been developed into a leaf-climber, if the foot-stalks had possessed in any slight degree the requisite sensitiveness to a touch. As twining is the simplest means of ascending a support, and forms the basis of our series, it may naturally be asked how did plants acquire this power in an incipient degree, afterwards to be improved and increased through natural se- lection. The power of twining depends, firstly, on the stems whilst young being extremely flexible (but this is a character common to many plants which are not climbers) ; and, sec-