6o4 THE STRUCTURE OF ECONOMIC PLANTS 



numerous cambial-like divisions which occur in a plane extending 

 tangentially across the axis. In the peg, the greatest dimension 

 of each parenchymatous cell is the radial one; while in other 

 parts of the hypocotyl, the long axes of the cells are in the vertical 

 plane. At the lower limits of the peg, this activity is usually 

 cortical; but in the upper part, the divisions may also involve 

 the stelar tissues which accounts for the abrupt outward divergence 

 in the course of the bundles at this level. The broad lower face 

 of the peg may bear root hairs and is oriented at right angles to 

 the axis; or, less frequently, at an acute angle to it. The upper 

 part of the peg merges gradually into the hypocotyl and has the 

 same smooth cutinized epidermis characteristic of the latter. 

 Above this point, the epidermis produces hairs and stomata, the 

 latter occurring in a frequency of about lo to 15 per sq. mm. 



Crocker, Knight, and Roberts (5) state that the development 

 of the peg is related to external factors which are concerned with 

 the arching of the hypocotyl, the contact of the seed coats, and 

 gravity. They express the opinion that there is no evidence that 

 gravity acts as a direct stimulus to the lateral placement of the 

 peg; and that 



"Contact of the coats is by far the more effective for it will induce 

 very sharp arching even against gravity. . . . Arching leads to an 

 increased development of the peg, as well as to its lateral placement, 

 and in many cases produces a peg where it would not otherwise appear 

 as in Big Tom. Contact likewise increases the size of the peg independ- 

 ent of its effect through arch production." 



The Hypocotyl. — The peg is at the base of the oval hypocotyl 

 which continues this configuration to the cotyledonary node, 

 the long axis being in the vertical plane of the cotyledons. The 

 course and number of the bundles in the hypocotyl is variable. 

 The eight original bundles are arranged two at each end of the 

 oval and two along each side (Fig. 313, H); but, usually, the 

 number increases just above the level of the peg. This is accom- 

 panied by branching and anastomoses of the eight bundles, so 

 that they may vary from 10 to as many as 16, the most common 

 number being 10 or ix. (Fig. 314, B.) 



In the upper portion of the transition zone, the two bundles at 

 each end of the oval anastomose, and each of the resulting large 

 bundles continues up the axis for a short distance and then is 

 divided to form • three bundles. (Fig. 313, I, /.) These six, 



