INTRODUCTION 25 



roots. The whole forms a root-system. The older 

 roots are furthest from the tip, the youngest the 

 nearest. Near the tip are the root-hairs used in 

 osmosis. 



In form roots may be tap roots growing con- 

 tinuously downwards, and becoming larger. When 

 the roots are swollen and fleshy they are tuberous. 

 Some roots are conical, some are napiform, or like 

 the turnip, others are spindle-shaped or fusiform. 

 The lateral roots may equal the primary root in size, 

 and these are fibrous. Adventitious roots develop 

 at the nodes of creeping stems, etc., and give rise to 

 new plants. 



Stems serve for support of the leaves and flowers, 

 and internally for water conduction, etc. Stems 

 differ in being herbaceous or woody, hence we get 

 herbs, shrubs, and trees. The former possess some 

 woody tissue, but it is less developed. Stems are 

 erect, prostrate, climbing or underground according 

 to the nature of the habitat. 



The characteristics of climbing plants are described 

 under Clematis in vol. ii. In twining plants the 

 stems turn to the right or to the left. Stems, for 

 protection or for adaptation to dry conditions, de- 

 velop thorns and spines. These are altered shoots. 



Stems may also grow underground in the form of 

 rhizomes, suckers, tubers, corms, bulbs. 



Stems may branch, the first axis or main axis 

 giving rise to branches or secondary axes, and these 

 in turn to tertiary axes. A stem which grows at 



