390 METAMORPHOSIS 



in the case of meadow plants, which are not infrequently subject to inunda- 

 tion, the special adaptation to aquatic life may have arisen a long time before 

 the capacity for adaptation had come to be inherited. 



Hitherto we have dealt with the so-called ' active ' adaptations only, but 

 there are also other adaptations which may be termed ' passive '. There are not 

 only plants which are able to adapt themselves, but also others which are adapted, 

 which exhibit a series of peculiarities which permit them to live under certain 

 environmental extremes. In addition to the genuine aquatics we have also 

 hygrophilous types (such as the Hymenophyllaceae), the numerous xerophytes, 

 and halpphytes (or salt plants), and a general consideration of such passive 

 adaptations leads us to the conclusion that these have arisen by a hereditary 

 fixing of active adaptations. In many liverworts we find that the shape 

 of the thallus is dependent on light, for it remains narrow in light of limited in- 

 tensity and broadens as the degree of illumination increases, thus exposing a 

 maximum surface at right angles to the incident ray. The flattened form of 

 the green assimilatory organ has, as we know, a definite purpose. In the lower 

 plants this is a case of active adaptation, whilst in the leaf -blades of the higher 

 plants the adaptation has become fixed by heredity. The same is true of the 

 roots of many epiphytic orchids, where in many species these organs become 

 flattened when exposed to light, although in other cases the flattening is always 

 present, even when the roots are grown in the dark (GoEBEL, 1898). Numerous 

 examples of the same kind might be quoted, especially in relation to the phe- 

 nomena of dorsiventrality and polarity, phenomena which although, perhaps, 

 they should not, strictly speaking, be classed among adaptations may be re- 

 ferred to here because they can in many cases be readily shown to be due to 

 external factors such as light and gravity, although in other cases they have 

 arisen without such stimuli. Thus VOCHTING (1886) showed that the flowers of 

 Epilobium angustifolium and of Hemerocallis fulya owe their dorsiventrality to 

 gravity and that they become radial when this unilateral stimulus is with- 

 drawn. Dorsiventrality appears in Amaryllis formosissima under all conditions, 

 however, and gravity merely renders the dorsiventrality more intense. 



As has been already said, a general comparison of examples compels us to 

 believe in the derivation of the passive from the active adaptations, hence 

 these active adaptations must be capable of transmission, and yet experiment 

 does not confirm this conclusion. Plants which have lived in the high Alps for 

 thousands of years, and which have adapted themselves to their surroundings 

 by taking on very characteristic forms, lose all these peculiarities when they are 

 cultivated in the plains below. Conversely, lowland plants transplanted to an 

 alpine habitat take on an alpine form but lose the adaptations which they thus 

 acquire when once more brought back to their original home (BONNIER, 1895). 

 In the same way, in cases where adaptations have been induced experimentally, 

 it is found that these are in no sense permanent, and that the seeds of plants 

 which have been cultivated for a long time under exactly similar external con- 

 ditions still retain complete power of adapting themselves. The gap between 

 practice and theory can at present be bridged by hypotheses only. Perhaps an 

 active adaptation induces a certain effect on the idioplasm, so that it disposes 

 it to repeat more readily this adaptation than any other. The initiation and 

 disappearance of adaptations take place often not in one generation but in the 

 course of several; the influences which have been operative in the first generation 

 obviously have still some effect in the second, and, if that after-effect be com- 

 bined with the new influences, at first there will be only a partial inheritance, and 

 a complete transmission only after several generations. We are also acquainted 

 with changes in the plant which outlast the stimuli which induced them, and 

 we have, in our discussion of periodicity, recognized after-effects, which may be 

 compared to a certain extent with the partial transmission postulated above. 



