372 STRUGGLE BETWEEN PLANT-COMMUNITIES sect, xvii 



of plants, as is well known to farmers. It seems also capable of inducing 

 distinctions in floral structure, as an increased supply of food leads to 

 the production of a larger receptacle, larger flowers, and more numerous 

 floral leaves (for instance, more numerous carpels in Papaver). 



Not only are external structural features affected, but also likewise 

 are internal ones : not only the length of roots and internodes, the size 

 and thickness or length of leaves, the greater or smaller production of 

 hairs, and so forth, but also the relative thickness of cortex and stele and 

 pith of axes, of palisade and spongy parenchyma of leaves, the depth of 

 epidermis, the thickness of cuticle, the number and volume of vascular 

 bundles, the amount of lignification, the volume of xylem, of tracheae, of 

 tracheids, and of mechanical tissue, the size of intercellular spaces, the 

 amount of chlorophyll, the development of stomata, of endodermis, and 

 so forth. 



The plant thus has a demonstrable faculty of reacting to external 

 influences in a varied manner. Sometimes one of its parts may be directly 

 affected, without the others undergoing any change. Even one and the 

 same leaf adapts itself differently in harmony with different surroundings ; 

 for example, the upper parts of the leaves of Stratiotes project above the 

 water and, inter alia, acquire stomata, and become less transparent as 

 well as darker green than the submerged parts. ^ 



There is not only plasticity of form but also plasticity of biological 

 character.'^ Gardeners know from experience that weakling plants are 

 more easily killed by frost than are other individuals of the same species. 

 Annual or biennial species may be induced by external influences to 

 become perennial. The times of resting, of foliation, of defoliation, and 

 of flowering may be changed. Cleistogamous flowers may be called into 

 being by cold and dull weather. A number of facts bearing on this matter 

 have been collected by Henslow.^ The metabolism of the plant is, as a whole, 

 subject to the laws of adaptation or self-regulation. Saccharomyces 

 adjusts itself to the presence or absence of oxygen ; and the turgor of 

 the root is similarly adjusted to the resistance encountered. 



Von Wettstein* in his valuable work upon seasonal dimorphism 

 expressed the opinion that regular mowing (therefore also regularly 

 recurring natural influences) caused the production of reputedly definite 

 forms which showed their own times of flowering and belonged to charac- 

 teristic types. The work of the same botanist on the assumption of an 

 annual habit by cultivated plants, including Phaseolus, should also be 

 consulted.^ 



Plants are not all equally plastic. Differences among species in this 

 respect depend sometimes on disposition that is due to their affinity, 

 sometimes on the stage of evolution at which the species or genus is 

 (certain genera, including Hieracium and Rubus, seem to be in a condition 

 of active evolution), and sometimes on the degree to which acquired 

 characters are fixed by heredity. Some will change more in one direction, 

 others more in another. Even individuals belonging to one species are 

 not all equally variable. 



^ Costantin, 1883-5. 



" See Vochting, 1893 ; Grabner, 1893, ? ^4^^ 5 Gobel, 1908. 



^ Henslow, 1894, 1895, 1908. * Von Wettstein, 1900. 



' Von Wettstein, 1897-8. 



I 



