10 THE QUANTITATIVE METHOD IN BIOLOGY 



unlimited number of years without any important change in its 

 observable properties. Leontopodium is suitable for cultiva- 

 tion in the plains, yet its properties (facies) are so deeply modi- 

 fied that it becomes hardly recognizable ; in other words, this 

 species is very PLASTIC (modifiable, facilis). (See § 14.) 



§ 13.— EXAMPLES OF PLASTICITY.— Po/yg-owwrn am- 

 phibium is a good example of a species with a living sub- 

 stance capable of numerous reactions according to the condi- 

 tions of existence, which bring about an astonishing diversity 

 in its observable properties. This species occurs (for instance, 

 in Flanders) in three distinct forms (VARIANTS). The ter- 

 restrial form or variant is (in Flanders) a common and rather 

 noxious weed in cultivated fields. Its stem is erect and rather 

 robust, 50 cm. to i m. high, with numerous spreading leaves. 

 The aquatic form or variant is common in ponds and watery 

 ditches. The stems are longer (2 m. and more), flexible, float- 

 ing ; the leaves are few, rather thick, spreading on the surface 

 of the water and differing considerably from the leaves of 

 the terrestial form by their facies and anatomical structure. 

 The xerophytic variant is not uncommon on rather arid, sandy 

 soil. Here the stems are more or less creeping, the leaves 

 being more like those of the terrestrial form but smaller. 

 Transitions between the three mentioned types frequently 

 occur ; for instance, in shallow or dried-up ponds, muddy 

 ditches, etc. The types are so very different from one another 

 by their facies and internal structure that one can hardly 

 realize that there is no hereditary difference between them (in 

 other words, that the living mixture is the same in each and 

 all). This is, however, beyond any doubt, for it is possible to 

 obtain the three types (variants) by bringing parts (with buds) 

 of one specimen under the various conditions of existence 

 mentioned above. 



Another remarkable example of plasticity is Ranunculus 

 sceleratus. In the terrestrial variant, which is common on 

 rather wet soil, the stem is erect, the leaves are comparatively 

 small, spread out and divided into segments. In the aquatic 

 variant the leaves are thicker, larger, less divided and spread 

 on the surface of the water. Their facies is so characteristic 

 that they may be discerned at first sight from any other sort of 

 floating leaves of our flora. Limiting ourselves to the study 

 of the terrestrial variant,^ we would neither suspect the possi- 

 bility nor guess at the properties of the floating leaves. These 

 are produced by reactions which take place only in water. 



The two preceding species are examples of what we call 

 ordinarily variable or polymorphic species. Here polymorphism 



^ In a number of floras the floating leaves are passed over in silence. 



