12 THE QUANTITATIVE METHOD IN BIOLOGY 



discover whether the mentioned polymorphism, with regard to 

 certain properties, depends on the existence of several sub- 

 species differing from each other by their hving substance, or 

 is a consequence of plasticity with regard to other properties. 

 As far as I know, we have exact information only with reference 

 to the colours black and green. In the meantime it is advisable 

 to collect facts and material without encumbering the botanical 

 nomenclature with more names. 



§ 14.— PLASTICITY {continued). SENECIO AND CHRYS- 

 ANTHEMUM. — It may happen that certain species are very 

 plastic with regard to a certain property, whereas in other 

 species, which belong to the same systematic group, this same 

 property seems to be independent of the conditions of existence. 



EXAMPLE : In certain species of Senecio the number of 

 marginal florets varies very Uttle. In S. jacobaa, for instance, 

 this number is 13.1 On the other hand, in other Com- 

 positae, this property is very variable. In Chrysanthemum 

 carinatum, for instance, the extremes are about 3 and 30 or 

 still more. In this plant the number of marginal florets may 

 be augmented or diminished by modifying the conditions of 

 existence : the highest figures are obtained (according to my 

 experiments) when the plants are grown under favourable 

 conditions (in good soil, at sufficient distances from each other), 

 whereas lower figures are observed when the conditions are 

 rather poor ; for instance, when the plants grow close together 

 or when the soil is poor (§ 128). 



The difference between the hving mixture of 5. jacobaa and 

 that of C. carinatum is disclosed not only by the reactions 

 which produce the so-called characteristics of each species, but 

 also by the fact that C. carinatum is very plastic with reference 

 to the above-mentioned property, whereas S. jacobaa is very 

 slightly plastic. 



The degree of plasticity is a sfecific property in itself. (See § n .) 



§ 15. — PLASTICITY {continued). PHILODENDRON 

 PERTUSUM.- — In certain cases a property described as being 

 in a high degree characteristic of a given species disappears 

 (becomes latent) under the influence of certain external causes. 



EXAMPLE : Philodendron pertusum bears large leaves 

 pierced with round holes, which give to this plant the name 

 of pertusum (perforated). In the Botanical Garden in Ghent 

 several specimens of this species were cultivated for many years 

 in a greenhouse which was rather cool and dry. The holes were 



1 In Flanders, I have counted the marginal florets of thousands of flower- 

 heads in many localities, under very different conditions of existence. I 

 always found 13, except in three or four flower-heads in which the figure 

 was 12 or 14. 



