298 THE ROOT 



strands on each side of the point of origin. In this 

 way the conducting systems of the mother root and of 

 the branch roots become part of one system. The 

 cortex of the branch, however, never has any con- 

 nexion with the cortex of the mother root. 



(2) Secondary Thickening. In perennial plants which 

 have a persistent primary root system, and also in 

 many annuals, the primary tissues are added to by 

 the activity of a secondary meristem. In other words, 

 certain living cells of the primary tissue begin to divide, 

 and the products of their division become differentiated 

 to form new permanent tissues, which are called 

 secondary tissues, especially new (secondary) xylem and 

 phloem. This process takes place in the roots and 

 stems of all the woody plants, and to a certain extent 

 in most herbaceous plants belonging to the Dicoty- 

 ledons (the largest group of seed plants) and to the 

 Conifers (pines, firs, etc.). The secondary vascular 

 meristem is called the cambium, and its activity results 

 in the great and continual increase in thickness which 

 occurs in the stems and roots of trees. We shall have 

 to consider it again in connexion with the woody stem, 

 but we may here note the beginning of the process 

 in persistent roots. 



The formation of the cambium starts by the tangential 

 division of the conjunctive cells just inside and on the 

 flanks of the primary phloem strands. The cells cut 

 off on the inside, i.e. towards the centre of the root, 

 become new (secondary) xylem, the strands so formed 

 alternating with the primary xylem strands (Fig. 49, E). 

 A few sieve tubes are cut off on the outside of the 

 cambium and added to the primary phloem. The 

 cambium now extends round the outside of the primary 

 xylem strands so as to form a complete layer round 



