INTRODUCTION 23 



between this group and other orders are mainly 

 structural, in the relation of the parts of the flower. 



We may thus say that the vegetative parts of 

 plants are more easily altered by the environment 

 than are those structures concerned with reproduc- 

 tion, and possessed of some inherent character not 

 influenced by environment (at least not so easily) 

 which is transmitted through the germ-plasm. Here- 

 dity is stronger than adaptations of the soma to the 

 environment. 



There are several types of habit or growth form. 

 In the early days of botany these were limited to 

 trees, shrubs and herbs, and such forms were used 

 in classification. But we now know that trees may 

 belong to a number of different orders which have 

 the tree habit in common. 



It is true that we may recognise many species by 

 their habit at a distance, e.g. the heaths, grasses, 

 etc., each having the heath or the grass habit, as 

 trees have the tree habit. But many other plants 

 have the heath or the grass habit which do not 

 belong to the heath order or the grass order. 



Amongst flowering plants there are what are called 

 heterotrophic plants which include saprophytes and 

 parasites that live upon decaying vegetable matter 

 and are not green, as the Bird's Nest, and parasites 

 that live upon other plants, as Broomrape. These 

 also are not green plants. 



There are also lianes which rely upon other plants 

 for growth support, and are generally climbers. Such 



