ADAPTATION 951 



roots commonly are hairless and wheat roots hairclad, certain maize 

 roots may be hairclad or wheat roots hairless, though the conditions 

 appear to be the same. 



Congenital and reaction structures. Structures (such as cork or 

 cutin) which arise through reaction to environmental changes may be 

 called reaction structures. 1 If a plant, when placed in xerophytic con- 

 ditions, happens to develop as a reaction thereto such xerophytic features 

 as cutinization, succulence, or dwarfness, it may be called a reaction 

 xerophyte; similarly, there may develop reaction mesophytes or reaction 

 hydrophytes. It is obvious, however, that many reaction structures 

 cannot be classed as hydrophytic, mesophytic, or xerophytic ; especially 

 is this true of those that do not happen to be advantageous. Contrasting 

 with reaction structures are those structures that are born with the 

 species in whatever habitat it is developed (as in the case of mutations), 

 and which are not lost if the species is grown in other habitats. Such 

 structures may be termed congenital structures. If a species happens to 

 be born with such xerophytic features as succulence or dwarfness, it may 

 be called a congenital xerophyte; similarly, there may develop congenital 

 hydrophytes or congenital mesophytes, or plants that cannot be thus 

 classed. 2 Thus any given structure, as a cutinized epidermal wall or 

 a succulent cortex, may be a reaction structure in one species and a con- 

 genital structure in another species. Furthermore, any species may at 

 the same time possess both variable reaction structures and fixed con- 

 genital structures, otherwise called adaptation and organization charac- 

 ters, careful experiment alone determining which is which. According 

 to this theory, each plant association is composed of certain species that 

 are fit because their critical features are the product of the habitat, and 

 of other species that happen to have been born fit and thus enabled to 

 survive. 



Even congenital structures and organs are influenced by external 

 factors. It has been seen that most stomata, hairs, and spines, if present 

 at all, have a definite and fixed structure, and thus may be regarded as 

 congenital rather than reactive. However, their presence or absence is 

 determined by external agents, whence the latter are called determinative 



1 Too much emphasis cannot be placed upon the fimdamental distinction between the 

 word reaction and such words as adaptation, adjustment, accommodation, or regulation. 

 The latter words imply an inherent power to change advantageously, while the word 

 reaction implies no such power. 



2 Reaction and congenital xerophytes, mesophytes, and hydrophytes also may be termed, 

 respectively, facultative and obligate xerophytes, mesophytes, and hydrophytes. 



