"Climbing plants may be divided into four classes. First those which twine spirally round a support and are not aided by any other movement. Secondly those endowed with irritable organs which when they touch any object clasp it; such organs consisting of modified leaves  branches or flower-peduncles. But these two classes sometimes graduate to a certain extent into one another. Plants of the third class ascend merely by the aid of hooks; and those of the fourth by rootlets; but as in neither class do the plants exhibit any special movements they present little interest and generally when I speak of climbing plants I refer to the two first great classes.