36 



MORPHOLOGY OF THE ANGIOSPERMS 



shorter than that of the shoot and differs from it in its clearer and 

 more constant zonation, in the definition of layers close to the apex, in 

 the presence of a root cap, and in the absence of primordia of ap- 

 pendages. 



The meristematic layers of the root apex of angiosperms are usually 

 formed by three, rarely four, groups of initiating cells (Fig. 13). In the 

 dicotyledons, the distal group forms the cap and the dermatogen; the 

 median group, the periblem; the innermost, the plerome. In the mono- 

 cotyledons, the distal group forms the cap; the median, the dermatogen 



Fig. 13. Diagrams of root-apex types in 

 angiosperms. A, initials in three groups, 

 cap not distinct, formed by same initials 

 as dermatogen; B, initials in three groups, 

 cap distinct in structure and independ- 

 ent in origin. {After Eamcs and Mac- 

 Daniels. ) 



Fig. 14. Median longitudinal section of 

 the basal end of a mature embryo of 

 Tropacolum majus, showing initial cell 

 (Z) and suspensors attached at the 

 base. (After von Guttenhcrg.) 



and periblem; the innermost, the plerome. The outstanding characteristic 

 of the apex of the dicotyledon root is the common origin of cap and 

 dermatogen, a resemblance to the ancient type of root-apex origin, 

 where both cap and epidermis are formed by a solitary apical cell 

 (Fig. 14). 



The root apex, lacking the complexities of appendage development, 

 is simple, though the cap adds a different "appendage." Its simplicity of 

 structure suggests that it is more primitive than the shoot apex. The 

 simplicity of the root apex adds to the evidence (lack of appendages, 

 protostelic structure, and exarch xylem ) that the root is a more primitive 

 organ than the shoot. 



