DEFINITIONS OF THE ORGANS 139 



relation to the clusters of primary phloem in all probability because 

 the foodstuffs are provided by that tissue. The active layer 

 which makes its appearance here continually adds new elements 

 on the outside to the phloem, and on the inside gives rise to the 

 secondary xylem. As a consequence of this situation the secondary 

 wood, being formed opposite the clusters of primary phloem, 

 naturally alternates with the primary wood. With the beginning 

 of the secondary growth the root therefore abandons the radial 

 type of organization of the primary structures. The lateral roots 

 originating from a given root have their position determined, 

 however, by the topography of the primary structures in the main 

 root and grow out in vertical rectilinear rows corresponding to the 

 primary xylem. But in the monocotyledons an exception to this 

 condition is found, since each lateral root originates in the interval 

 between two angles of the primary xylem. 



Although the secondary structures of the root, as has been 

 indicated above, are collaterally organized, the primary plan of the 

 root is radial. The root, in fact, is the only organ of the plant 

 for which a single ground plan will illustrate the conditions found 

 in all groups of plants, living or extinct. The extreme conservatism 

 of the anatomical organization of the root is an outstanding feature 

 and, as will be indicated in succeeding chapters, renders it of the 

 greatest value in working out the evolutionary sequence of the 

 various groups. Extreme conservatism, radial organization of 

 the primary fibrovascular structures, the possession of a root cap 

 or pileorhiza, and an internal origin from the surface of the fibro- 

 vascular cylinder are the salient and important criteria of the root. 



THE STEM 



In the case of the axis or shoot of plants no such general formula 

 can be arrived at as in the root. Just as the root is the least change- 

 able of all the organs of the plant, so the stem is of all the most 

 variable. In internal organization it varies greatly from the lower 

 and more ancient groups to the higher and more modern. So 

 numerous are the types of stem as regards anatomical structure 

 that they can most profitably be discussed under the particular 

 groups of the vascular plants in later chapters of the volume. In 



