374 ADAPTIVE VARIATIONS. 



in the duration of the exposure. In most animals, the 

 change in the direction of adaptation is slight, but it is 

 probably always there, if only it be carefully looked 

 for. In plants it is, as a rule, greater, and may be ob- 

 vious to the most cursory observation. Instances of 

 it have already been described at some length in pre- 

 ceding chapters, and hence it is unnecessary to do more 

 than briefly recall these here. We saw that Karsten 

 found that a kidney bean reared in the dark for a 

 month or two weighed 20 per cent, more than one 

 reared in the light, yet, owing to the absence of the 

 stimulus of light, its leaves did not weigh a fifth as 

 much. Lothelier found that plants such as Berberis 

 vulgaris bore non-spinescent leaves in a moist atmos- 

 phere, but spines and spines alone in a perfectly dry 

 one. Costantin found that he could change the form 

 of Hippuris at will, by growing the aquatic form of the 

 plant on land, and the terrestrial form in water. All 

 the leaves produced under water were long, undulated, 

 and delicate, whilst those in air were short, erect, and 

 firm. Costantin, and also Godron, obtained very simi- 

 lar results by growing other aquatic plants on land, and 

 terrestrial ones in water, the change being always in 

 the direction of adaptation to the new surroundings. 

 Again, Lesage found that by watering various plants 

 with water containing salt they developed characters 

 similar to those exhibited by maritime plants, viz., in- 

 creased thickness of leaves, larger and more numer- 

 ous palisade cells, and diminution of. the intercellular 

 spaces and of the chlorophyll. Bonnier found that 

 plants of Teucrium scorodonia, when grown at a high 

 situation in the Pyrenees, exhibited features character- 



