40 THE STRUCTURE OF ECONOMIC PLANTS 



development of roots. Weaver (13) and Weaver and Bruner (14) 

 have described and discussed the roots of the principal field and 

 vegetable crops in connection v^ith these points. 



Gross Morphology. — The root axis differs in certain anatomi- 

 cal respects from that of the stem so that generally it can be dis- 

 tinguished from the latter on the basis of its structure. A 

 representative root in which the differentiation of primary tissues is 

 complete has three w^ell-defined regions, epdermis, cortex, and 

 stele. The epidermis consists of a single layer of thin-walled cells 

 which are devoid of cuticle and have the potentialities to form 

 root hairs by a lateral extension of their outer walls. The number 

 and character of development of root hairs depend upon the 

 species involved and the soil conditions. Centrad to the epidermis 

 is the cortical region, consisting of several layers of parenchyma- 

 tous cells that are limited internally by a uniseriate endodermis 

 which is regarded as a part of the cortex. The endodermal cells 

 have Casparian thickenings that are usually more clearly defined 

 than those of the endodermis of most stems. At maturity, the 

 inner tangential, radial, and end walls of some of the endodermal 

 cells may be appreciably thickened; while others, which lie 

 approximately outside the protoxylem points, may have their 

 walls thickened to a lesser degree. 



The primary root of dicotyledons typically has a radial proto- 

 stele; or, less frequently, a radial siphonostele. The former con- 

 sists of a central mass of primary xylem tissue with two to several 

 radiating arms extending centrifugally to the pericycle. The lat- 

 ter constitutes the outermost layer of the stele, and lies immedi- 

 ately adjacent to the endodermis. At the time of maturation of 

 the primary tissues of the stele, the pericycle is commonly a single 

 layer of cells which form a continuous cylinder; but, in some in- 

 stances, it may be interrupted by the protoxylem cells which di- 

 rectly abut the endodermis. In some roots, the pericyclic cells be- 

 come active and undergo division prior to the complete maturation 

 of the vascular tissues, and this frequently happens where there 

 is an early differentiation of lateral roots. 



Between the radiating arms of xylem are located the two to 

 several groups of primary phloem; and, between the xylem and 

 phloem, there may be a zone of parenchymatous cells from which 

 a cambium may be differentiated. The number of protoxylem 

 strands is fairly constant for a given species, but some variation 



