594 



THE STRUCTURE OF ECONOMIC PLANTS 



protophloem ducts similar to the initial one may be differentiated 

 adjacent to the pericycle or one or two cell layers removed from it. 



The metaphloem develops sieve tubes and companion cells in 

 addition to parenchyma. The sieve tubes are elongated, small in 

 diameter, and have comparatively small sieve plates. The com- 

 panion cells are nucleate, densely cytoplasmic, and shorter than 

 the sieve tubes so that there may be two or more of them abutting 

 one sieve tube. The major portion of the metaphloem is composed 

 of parenchymatous cells that are larger in diameter than the sieve 

 tubes and usually several times longer than their radial dimension. 



The differentiation of the protoxylem parallels the protophloem. 

 The first elements which mature have relatively heavy annular 

 thickenings, and one vessel of this type develops at the outer 

 limit of each protoxylem point. The two or three protoxylem 

 elements centrad to the first formed, annular elements, differentiate 

 as annular or spiral vessels. The relative number of each type is 

 not constant, and occasionally intermediate spiral-annular elements 

 may be formed. The protoxylem vessels soon become stretched 

 and crushed, forming lacunae which may later become conspicuous. 

 The tetrarch pattern of the stele becomes more marked with the 

 progressive differentiation of the metaxylem elements. There are 

 intermediate types between the spiral, reticulate, and scalariform 

 patterns; so that it is difficult to distinguish definitely between 

 the protoxylem and metaxylem. In general, the metaxylem 

 vessels are conspicuously larger than the first formed protoxylem 

 elements, and, for the most part, have closely reticulated thicken- 

 ings. 



Up to this point, the stelar development results in the formation 

 of four radial arms of primary xylem alternating with the four 

 zones of primary phloem, the central portion of the axis remaining 

 parenchymatous. (Fig. 310.) Possibly because of the delayed 

 differentiation of the central portion of the stele, the squash root 

 has been described as having a pith. Van Tieghem (35) and 

 Gerard (ix) both mention the presence of a pith; but in the latter 

 case the transection described was probably close to the transition 

 region where a pith is differentiated. Rutledge (34) and Whiting 

 point out that, in C. maxima, two or more cells at the center of 

 the stele finally develop to form conspicuously large pitted or 

 reticulate-pitted tracheae. These are surrounded by smaller, 

 isodiametric or horizontally elongated parenchymatous cells which 



