44 THE STRUCTURE OF ECONOMIC PLANTS 



regions of the root axis. Further back is the region of maturation , 

 in which the primary tissues attain the characteristics which they 

 possess at maturity. The designation of regions of elongation, 

 differentiation, maturation, and maturity in describing the ontog- 

 eny of the root axis is one largely of convenience, and there are 

 of course no sharp boundaries delimiting these zones. There may 

 be continued cell division in certain tissues of the developing axis, 

 while the adjacent cells of some other tissue are compensating for 

 this by cellular enlargement. In addition, some cells, even in the 

 mature region, retain meristematic potentialities which may or 

 may not later be expressed. 



The Histogens. — The primary meristem at the tip of the root, 

 where cell division is going on most actively, may itself be dif- 

 ferentiated into regions. These are known as histogens; and, in 

 ontogeny, the cells derived directly from them can be specifically 

 related to the mature regions of the root. Hanstein (5) designated 

 three of these regions as the dermatogen, peribletn, and pleromej and, 

 later, Janczewski (6) added a fourth, the calyptrogen. The latter 

 term is applied when the root cap is derived from and maintained 

 by the activity of a definite layer of meristematic cells distal to the 

 other cells of the growing point. In cases where it is not possible 

 to separate the meristematic cells that give rise to the root cap from 

 others in the growing point, no specific designation is made except 

 when the epidermis and root cap arise from a common layer. In 

 this instance, the term dermatogen-calyptrogen has been applied. 



The dermatogen, when distinct from the calyptrogen, consists 

 of a single layer of cells underlying the latter, which perpetuates 

 itself and produces epidermal cells by continued anticlinal divi- 

 sions. The periblem is a generative layer inside the dermatogen 

 which gives rise to the tissues of the cortex. In most cases, this 

 histogen is one layer in thickness at the apex of the root; and, 

 like the other histogens, it maintains itself by anticlinal divisions. 

 The lateral cells of the periblem divide periclinally, so that the 

 zone becomes several layered; and, subsequently, divisions of the 

 derivative cells in three planes result in the formation of the cortical 

 parenchyma and endodermis. The innermost histogen or plerome 

 produces the stelar elements many of which are elongated; and the 

 derivative cells divide transversely less frequently than do those of 

 the cortex. This difference in the mode of cell division in the two 

 adjacent regions derived from the periblem and plerome results in 



