RAPHANUS SATIVUS 195 



proceeds, the diarch, radial protostele, cortex and epidermis may be 

 clearly distinguished approximately one millimeter from the tip of 

 the root. At this level, the epidermal cells are nearly isodiametric 

 and the cortical parenchyma consists of thin-walled cells which 

 have formed many intercellular spaces. The cells of the endoder- 

 mis are small and compactly arranged, with no intercellular spaces. 

 Before there is any differentiation of the secondary cell walls of 

 the primary xylem, the centrally located metaxylem vessels can be 

 distinguished by their larger caliber, as well as the elongated 

 parenchymatous elements of the protophloem. 



The protoxylem develops centripetally and the first elements 

 differentiate from the procambial cells abutting the pericycle as 

 spiral vessels. The metaxylem vessels are scalariform or reticulate, 

 some of the latter approximating the pitted type. The primary 

 xylem and phloem are separated by a zone of parenchymatous 

 cells from which the cambium is later differentiated. The pericycle 

 is at first a uniseriate layer; but very early in ontogeny becomes 

 multi-layered as a result of periclinal divisions. At this stage, 

 the epidermal cells elongate slightly in their axial dimension, the 

 cortical cells enlarge in all three dimensions, keeping pace with 

 the increase in the size of the stele, and the cells of the endoder- 

 mis develop well-defined Casparian strips on their radial and end 

 walls. 



Lateral Roots. — The initiation of lateral roots is coincident 

 with the maturation of the large, reticulate, metaxylem elements. 

 Their point of origin is definitely related to the diarch structure 

 of the stele since the root primordia arise in the pericyclic region 

 outside the protoxylem points or in some cases slightly tangent to 

 them. This results in the formation of two rows of lateral roots 

 which appear to arise from longitudinal furrows. These are not 

 present at the initiation of lateral root formation, but result from 

 the less rapid development of the tissues in the plane of the second- 

 ary roots. The number of secondary roots may be large, depending 

 upon cultural conditions, but they are limited for the most part to 

 the lower portion of the fleshy tap root, very few being formed in 

 the upper hypocotyl. The structure of the lateral root resembles 

 that of the non-fleshy portion of the primary root. 



The Hypocotyl. — Except for a zone x or 3 mm. in length, the 

 hypocotyl elongates rapidly in the young seedling owing to con- 

 tinued division and enlargement of the cells. There is little struc- 



