22 THE STORY OF PLANT LIFE 



accomplished artificially by artificial pollination, as 

 in the case of varieties or forms. For a discussion 

 of Mendelism and the principles of plant-breeding, 

 the works cited in the Bibliography (vol. ii) should 

 be consulted. 



13 . The Forms of Plants, and Adaptation to 

 Environment, and Function. 



One may either consider the forms of plants, 

 taking into account the plant as a whole, or the 

 forms of the separate parts of the plant or its organs. 



Generally speaking the growth-form exhibited by 

 each plant is an expression of its adaptation to the 

 environment, to a very large extent. The influence 

 of heredity and the trend of evolution, of course, in 

 the first instance regulates such forms. But though 

 a large number of plants of the same group exhibit 

 the same type or habit, as the cushion habit in the 

 Stonecrops, yet there are numerous orders in which 

 different types of habit in the same genus are repre- 

 sented, as in the case of the Buttercups, where the 

 yellow-flowered forms are land plants with an erect 

 or creeping habit, broad leaves and short internodes, 

 whilst the white-flowered types are aquatic plants, 

 and have narrow leaves and often a streaming habit 

 — with long internodes, and a different mode of 

 nutrition. But all the Buttercups agree in certain 

 characters which causes them to be placed in the 

 group Ranunculacese. The differences that occur 



