134 



THE ANATOMY OF WOODY PLANTS 



belonging to the fundamental system can readily be distinguished. 

 It may be stated in a general way that the higher vascular 

 plants have their skeletal tissues developed from the progressive 



mechanical differentia- 

 tion of the primitively 

 solely water-conducting 

 fibrovascular system, in 

 contrast to the Pteri- 

 dophyta, in which the 

 mechanical function 

 mainly resides in the 

 fundamental system. 

 In the higher vascular 

 groups not only does 

 the mechanical principle 

 find its best expression 

 in connection with the 

 fibrovascular system, 



FIG. 98. Transverse section of young root 

 of the balsam fir. Explanation in the text. 



but the fundamen- 

 tal category of 

 tissues becomes 

 relatively insignifi- 

 cant in amount, 

 particularly in 

 stems with peren- 

 nial growth. Fur- 

 ther, the bound- 

 aries between the 

 fibrovascular and 

 fundamental tis- 

 sues have gen- 

 erally disappeared 

 through the de- 

 generacy of the 

 endodermis. 



In the case of the root of all vascular plants the limit between the 

 conductive strands and the fundamental system is clearly marked 

 from the lowest forms to those at the very summit of the vascular 



FIG. 99. Older root of the balsam fir 



