132. THE STRUCTURE OF ECONOMIC PLANTS 



differentiated in roots that are less than forty-eight hours old. The 

 endodermis of the primary axis persists as a continuous layer over 

 the lateral root primordium until it is well differentiated and 

 has reached the outer limits of the cortex. The point of origin 

 of the lateral roots is approximately on the same radius as the 

 protoxylem strands; and, according to van Tieghem and Douliot 

 (x8), the axis of the new root usually deviates 15° to xo° from this 

 plane. The ontogeny of the lateral root is similar to that of the 

 primary root, and this is also true of adventitious roots which may 

 arise in the hypocotyl. 



Vascular Transition. — The structure of the hypocotyl is root- 

 like from the ground level to a point about 1.5 cm. below the 

 cotyledonary node, except that the central portion of the axis 

 becomes parenchymatous. This condition begins about a milli- 

 meter above the ground level, where the central portion of the 

 axis is comprised of sclerenchymatous cells rather than metaxylem. 

 At successively higher levels, the cells of the central parenchyma 

 have thinner walls, and immediately below the cotyledonary node 

 there may be a complete disintegration of the pith in plants that 

 are not more than two weeks old. 



The character of the metaxylem elements also changes, these 

 being differentiated with spiral thickenings in the upper hypocotyl; 

 and vessels with bordered pits, such as occur in the metaxylem of 

 the root are rarely found. The pericycle is multilayered as in the 

 root, the number of layers ranging from three to five outside the 

 protoxylem points, with two layers in the intercotyledonary plane. 

 The endodermis is not clearly defined, and the cortex of seven or 

 more layers is limited outwardly by a definite hypodermis. 



The first modification of the root-like organization of the stelar 

 tissue occurs about 1.5 cm. below the cotyledonary node where 

 the metaxylem is differentiated tangentially with reference to 

 the protoxylem points. This results in the formation of two 

 V-shaped groups of primary xylem in which the protoxylem 

 occupies the apex of the V and the metaxylem the extended arms. 

 Each phloem group lies adjacent to one of the lateral metaxylem 

 groups, and this relationship is maintained throughout the entire 

 transition region. 



As the reorientation of the metaxylem proceeds, a vascular 

 strand is diverged from each arm of metaxylem toward the inter- 

 cotyledonary plane; and the two strands on either side of the inter- 



