26 MERISTEJVIS 



preted in terms of the "histogeii theory" (Hayward, 1938, pp. 

 44-48; vou Guttenberg, 1940). While it is true tliat the absence 

 of foliar structures in roots makes it relatively easy to determine 

 the point of origin of a given tissue, it may well be questioned 

 whether Hanstein's concepts are any more justified for the root 

 than for the shoot. It seems evident, at any event, that a broad 

 systematic survey of the structure and behavior of the root apex 

 in angiosperms and gymnosperms would remove the problem from 

 the highly fortnalizcd i)o<sition which it now occupies. 



Secure serial longitudinal and transverse sections of some of 

 the principal "types" of root apices, viz.: (1) the "grass type," 

 characterized by the possession of a discrete meristem termed the 

 calyptrogeii which exclusively propagates the root cap, and by 

 the origin of "dermatogen" and "periblem" from a common 

 initial group; (2) the "Allium type" in which, according to 

 Ilayward (1988, p. 46) "the root cap, epidermis, and cortex 

 arise from a common group of initials two cell layers in thickness, 

 and within this zone is a sharply defined plerome;" (3) the 

 " Helianthus" type which is believed to represent the most com- 

 mon type in dicotyledons, and in which "the plerome and 

 ])eriblem are sharply defined; and outside the latter is a common 

 initial layer which produces the root cap and the epidermis" 

 (Ilayward, 1938, p. 46, and Ilaberlandt, pp. 89-90, Fig. 19) ; 

 and (4) the "Pisum" type in which a transverse initiation zone 

 is the common point of origin of root cap as well as the primary 

 tissues of the root. In this type, which is found in Cucurhita 

 and many Leguminosae, well-defined "histogens" are not recog- 

 nized (Ilayward, p. 47, Fig. 17). Regardless of "type," careful 

 inspection of median loiigi-sections under low and high magnifi- 

 cation will reveal the successive "zones" of cell origin, cell elon- 

 gation, and cell maturation. The "rih meristem" of the outer 

 peripheral region of the root tip is particularly useful in study- 

 ing these zones, because of the extremely regular arrangement 

 of the cell rows and the gradual changes in size, shape, wall thick- 

 ness, and degree of vacuolat ioii of Ilu> coihijoiumiI cells. Note in 

 contrast the relatively short "zone of transition" from the 

 "calyptrogen" (or its equivalent) to the outer senescent cells of 

 the root cap. The marked differences in the rate and duration of 



