384 THE STRUCTURE OF ECONOMIC PLANTS 



metaxylem elements differentiate centripetally until all the elements 

 of the xylem plate are lignified and no central parenchyma remains. 

 The outermost metaxylem elements have scalariform wall thickenings 

 while those centrally located have pitted wall thickenings." 



The Origin of Lateral Roots. — The lateral roots are initiated 

 at about the time that the secondary wall thickenings of the 

 metaxylem elements are being laid down. The first secondary 

 root primordia occur just below the soil level and are oriented in 

 the plane of the protoxylem strands or more frequently slightly 

 to one side of them. The cells of the pericycle adjacent to the 

 protoxylem points undergo tangential divisions which result in the 

 production of two distinct layers. (Fig. 196, A, a and ^.) The 

 cells of each of these layers divide tangentially so that the root pri- 

 mordium is four layers in width as seen in a transection of the 

 primary root. (Fig. 196, B.) The divisions of the first two 

 layers do not take place simultaneously, and ordinarily the inner 

 layer divides somewhat in advance of the outer one. In order to 

 compensate for the increasing length of the overlapping layers, a 

 few radial divisions occur in the root primordium but these take 

 place in such a way that they cause no distortion of the tangential 

 alignment of the rows of cells. The four layers, derived from the 

 pericycle, function as histogens: the outer one a' becomes the 

 calyptrogen-dermatogeA layer; the next one a" functions as the 

 periblem; and the two inner ones b' and b" give rise to the plerome. 

 While the growing point of the lateral root is differentiated 

 from pericyclic derivatives, the adjacent cells of the endodermis 

 divide radially to compensate for its enlargement. This anticlinal 

 division of the endodermal cells continues so that it persists as a 

 single layer of cells about the tip of the lateral root. (Fig. 196, 

 £-H.) It never becomes more than one cell layer in thickness, but 

 maintains its position until the lateral root penetrates through the 

 cortex of the primary root and into the soil, when it disintegrates, 

 together with the outermost layer of the root cap. As the lateral 

 root penetrates the cortex, there is little accumulation of debris or 

 distortion of the cortical cells, which indicates that the emergence 

 of the lateral root involves some digestion as well as mechanical 

 crushing of the cortical tissues. 



In tracing the subsequent development of the histogens, Crooks 

 has pointed out that the two inner layers constituting the plerome 

 continue to divide in all planes and produce a conical group of cells 



