ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 671 



or of an athlete increase mth eifort and decrease with disuse, so do 

 plants develop tissues under the influence of strains, &c., which best 

 enable them to meet the various forces to which they may be subjected. 

 On the other hand, such tissues are more or less arrested when no 

 strains are present. 



It is in accordance with this property of plants that we find pro^ 

 jecting angles and columnar structures generally composed of coUen- 

 chyma on the surface of erect herbaceous stems, as in the LahiatcB and 

 UmbellifercB : pericycular sclerenchyma in the flower-stalks of Carna- 

 tions, Ixias, &c., and huge buttresses to some foreign tropical trees. 

 (Figs. 274, 275.) 



The general conclusion arrived at from the study of the forms and 

 distributions of supportive tissues is that they are perfectly analogous to 

 what takes place in animals. Prof. Haughton has shown, in his lectures 



Fig. 276. — Solanumjasminoides, with one 

 . petiole twined round a support and 

 thickened. (After Darwin.) 



on " The Principle of Least Action," how the muscular arrangements 

 and structure and the forms of their supporting bones in animals all 

 follow this law ; that is to say, whatever force is required, the machinery 

 has been evolved to meet the case. I would venture, therefore, to lay it 

 down as universal in the living world, that the necessary structures in 

 both kingdoms have been evolved as Lamarck supposed them to have 

 been effected, through use and in response to effort. 



Experimental and Natural Proofs of Adaptation. — As an illustration 

 derived from experiments with plants, M. R. Hegler showed that an 

 increase of strength and development of the mechanical tissues of plants 

 resulted from the application of artificial strains produced by weighted 

 strings. Thus, the hypocotyl of a seedling Sunflower, which would have 

 been ruptured by a weight of 160 grms., bore a weight of 250 grms. 

 after having been subjected for two days to a strain of a weight of 



