102 STRUCTURAL BOTANY 



roots that are able to take up food from the soil is 

 constantly becoming greater. 



Thus we see how the growth of all the vegetative 

 organs of the plant works together. As the tran- 

 spiring and assimilating surface of the leaves increases, 

 the amount of conducting tissue in the stem and 

 older roots increases at an equal rate, while at the 

 same time the branching of the younger roots pro- 

 vides for the greater demands on their absorptive 

 capacity. 



Secondary growth in thickness is important in 

 other ways also : as the branching of the shoot goes 

 on, it is obvious that the stem has an ever-increasing 

 load to bear, due not merely to the increased weight, 

 but also to the greater leverage of the spreading 

 branches. Hence the necessity for increased mechan- 

 ical strength, which is provided for by the formation 

 of additional wood from the cambium, and especially 

 by its fibrous elements. 



Lastly, the constant formation of new tissues has 

 the further advantage that old elements which have 

 become past work are replaced by young and active 

 ones. This applies especially to the phloem, in which 

 it is generally the newer layers which are alone active. 

 This point, however, is not so important in the Wall- 

 flower as in some other plants, especially trees, in 

 which the duration of life is longer. Secondary 

 growth, then, has three main functions: to increase 

 the conducting tissues, to increase the mechanical 

 tissues, and to replace old elements by young 

 ones. 



