We chose the transformation of leaf forms into one another as the starting-point of our observations, because the principle of metamorphosis appears here in a most conspicuous manner. This principle, however, is not confined to this part of the plant's organism. In fact, all the different organs which the plant produces within its life cycle - foliage, calyx, corolla, organs of fertilization, fruit and seed - are metamorphoses of one and the same organ.