TISSUES AND SIMPLE ORGANS. 95 



ig) The Structure of Roots. 



The noticeable difference between the internal structure of roots 

 and stems suggests the question : What are the physiological causes 

 which produce such a difference ? Only since Schwendener's 

 physiological studies of tissues have we been enabled to give an 

 approximately correct answer to this question. According to the 

 requirements of mechanical tension, we find a central arrangement 

 of the relatively firm elements (see discussion of mechanical tis- 

 sues). This centripetal tendency of mechanical elements governs 

 the arrangement of tissues in the root ; the absence of a medullary 

 tissue, or at least the reduction of the same to a minimum, is 

 thereby readily explained (teleologically, not causal-mechanically). 



Fig. 57.— Cross-section of the root (diagramatic). 



«, Bundle-sheath ; p, pericambium. The root is " tetrarch " ; between the four vascular bun- 

 dles and the four groups of sieve-tissue lie the wood-parenchyma and the mechanical cells. 



The fact must not be overlooked that the older and more centrally 

 located root-portions serve primarily for the conduction of water, 

 while the root-tip portions are specially qualified to take food sub- 

 stances from the soil. This predominating conducting function of 

 older root-portions requires anatomical adaptations which are most 

 suitably met by the compact, narrowed, central vascular bundle. 

 It is known that the primary cortex very often loses its leaves, in 

 which case there is in reality only a central bundle remaining. 

 Leaving the root-cap out of consideration for the present, there are 

 three cmatomical differences between the root and the stem. These 

 differences are as follows. 



