THE ANATOMY OF THE ROOT 



45 



a blocking off of the cortex and stele on the basis of cell shape; 

 and, in many cases, it is possible to delimit them very early in 

 ontogeny. 



The degree to which the meristem is specialized into the four 

 histogens is variable. Janczewski (6) has recognized four princi- 

 pal types of such meristematic differentiation in angiosperms. In 

 type (i), the meristem is definitely and sharply differentiated into 



Fig. 15. Longisection of the apex of the corn root showing the histogens: a, the plerome; 



b, periblem which gives rise by periclinal divisions to the epidermis, d, and the cortical cells, e; 



c, the calyptrogen which produces the root cap. 



the four histogens, plerome, periblem, dermatogen, and calyptro- 

 gen, from whose respective derivative cells the stele, cortex, epi- 

 dermis, and root cap are developed. This type is an exceptional 

 one, and does not occur in any of the economic plants later de- 

 scribed. It is reported for a few hydrophytic monocotyledons in 

 the family Hydrocharitaceae (Pistia and Hydrocharis). 



At the apex of the root in type (x), the plerome and calyptrogen 

 are sharply defined, and there is an intermediate layer between them 



