The effect of a change in the conditions of growth in producing diversity in the form of the leaf may be here alluded to. _Ficus stipulata_, a plant used to cover the walls of plant-stoves in this country, and growing naturally on walls in India, like ivy, produces leaves of very different form, size, and texture, when grown as a standard, from what it does when adhering to a wall. _Marcgraavia umbellata_ furnishes another example of a similar nature, as indeed, to a less extent, does the common ivy. Allusion has been already made to the occasional persistence of forms in adult life, which are commonly confined to a young state, as in the case of some conifers which present on the same plant, at the same time, two different forms of leaves. Mention has also been made of the presence of adventitious buds on leaves and in other situations. The leaves that spring from these buds are usually of the same form as the other leaves of the plant, but now and then they differ. Of this a remarkable illustration is afforded by a fern, _Pteris quadriaurita_, in which the fronds emerging from an adventitious bud are very different from the ordinary fronds. [Illustration: FIG. 178.--Portion of a frond of _Pteris quadriaurita_, with an adventitious bud, the form of the constituent foliage of which is very different from that of the parent frond.] =Dimorphism.=--This term, applied specially to the varied form which the flowers or some of their constituent elements assume on the same plant, is an analogous phenomenon to what has been above spoken of as heterophylly, and, like it, it cannot, except under special circumstances, be considered as of teratological importance. A few illustrative cases, however, may here be cited. Sir George Mackenzie describes a variety of the potato[367] (_Solanum tuberosum_), which produces first double and sterile flowers, and subsequently single fertile ones; the other portions of the plant do not differ much. _Stackhousia juncea_, according to Clarke, has mixed with its perfect flowers a number of apetalous blossoms destitute of anthers.[368]