138 THE ANATOMY OF WOODY PLANTS 



cap and a type of fibro vascular organization known as radial. 

 The root cap or pileorhiza clearly differentiates roots from other 

 structures in the plant which may happen under the stress of 

 environment to assume a subterranean mode of existence. More- 

 over, the root cap persists even in those cases in which the root 

 becomes aerial or aquatic in its habit. We need not devote atten- 

 tion to the mode of origin of the pileorhiza, or protective tip of 

 the root, as studies of this nature are at the present time of doubt- 

 ful morphological value, and the consideration of them in an ele- 

 mentary work is quite out of the question. All that need be stated 

 is that the cap occupies the tip of the root and is continually renewed 

 from behind, sometimes, but not invariably, by a well-defined active 

 tissue known as the calyptrogen. The protective cap is character- 

 istic of all roots from the lowest to the highest plants. There is 

 only one general condition under which this structure is ordinarily 

 lacking. In the fungus-infected roots or mycorrhizae of humus 

 plants, in which the fungal infection is superficial and does not 

 notably invade the internal tissues of the root, the root cap is 

 degenerate or absent. In the case of the haustoria of certain root 

 parasites, such as the dodder, not only is the root cap absent, but 

 also the general condition of degeneracy is so marked that frequently 

 the only criterion of the morphological value of the structure is its 

 internal or endogenous origin in the axis of the parasite. 



A salient feature of the organization of the root is furnished 

 by the fibro vascular tissues. Here the type of fibro vascular sys- 

 tem is that known as radial. In this condition the masses of 

 phloem, instead of surrounding the wood or xylem (a state 

 commonly found in the ferns and their allies) or lying just outside 

 of it in the same radius (seed plants) as in the stem and leaf, lie 

 in different and distinct radii. This mode of relative disposition 

 of phloem and xylem is responsible for the term radial as applied to 

 roots, and it is a very important characteristic of their organization. 

 Where the phenomenon of secondary growth in thickness is absent, 

 there is no subsequent modification of the general relations of 

 tissues in roots. In the case of roots with secondary growth, how- 

 ever, the situation changes with the appearance of the secondary 

 xylem and phloem . The secondary tissues begin to form in definite 



