CHAPTER XVIII 

 ADAPTATIONS 



MOST of the discussions which have been presented in the pre- 

 ceding chapters have dealt with the types of compounds, the kinds 

 of reactions, and the mechanism for the control of these, which are 

 exhibited by plants under their normal conditions for development. 

 The results of the evolutionary process have produced in the dif- 

 ferent species of plants certain fixed habits of growth and metab- 

 olism. So definitely fixed are these that in each particular species 

 of plants each individual differs from other individuals, which 

 are of the same age and have had the same nutritional advantages 

 and environmental opportunities for growth, by scarcely percep- 

 tible variations, if at all. Indeed, this fixed habit of development 

 makes possible the classification of plants into genera, species, etc. 

 While different species of plants, given the same conditions of nutri- 

 tion and environment, produce organs of the widest conceivable 

 variety in form, color, and function; within the same species, the 

 form and size of leaves, the position and branching of the stem, 

 the color, size, and shape of the flower, the coloration and markings 

 of the fruit, etc., are relatively constant and subject to only very 

 slight modifications. 



It is unnecessary to say that the mechanism, or the impulses, 

 which govern the morphological characters of the tissues which any 

 given species of plants will elaborate out of the crude food material 

 which it receives from the soil and atmosphere, are wholly unknown 

 to science. It is the commonly accepted assumption that the fixed 

 habit of growth of the species is transmitted from generation to 

 generation through the chromosomes of the germ cells. But the 

 nature of the elements, or substances, which may be present in 

 the chromosomes, which influence the character of the organs 

 which will develop months later, after the plant which grows from 

 the germ cell has gone through its various stages of vegetative 

 growth, is still altogether unknown. There can be no question, 



249 



