304 THE STRUCTURE OF ECONOMIC PLANTS 



Food Reserves. — There is little stored food in the radish, 

 the bulk of the fleshy axis being water and cellulose. Some reserves 

 are accumulated in the thickened hypocotyl in the form of glucose 

 and there is a small amount of fructose. In certain Chinese types, 

 there may be a considerable amount of starch; but the French, 

 Japanese, and American varieties contain little if any under normal 

 conditions. In this connection, Plitt (13) found that slightly 

 thickened hypocotyls grown at high temperature with a long day 

 contained many small starch grains in the parenchyma. 



Anatomy of the Mature Stem. — The mature stem is sub-terete 

 or somewhat lobed and fluted in transection. The vascular bundles 

 are collateral and form a dissected siphonostele, but adjacent 

 bundles may be connected by a weak interfascicular cambium at 

 maturity. In addition to this, the thickening of the parenchyma- 

 tous cells surrounding the bundles results in the formation of a 

 practically uninterrupted zone of connective tissue giving the 

 vascular ring the appearance of complete continuity. The bundles 

 vary in size with the larger ones occupying the arcs of the vascular 

 ring centrad to the lobed or ridged portions of the stem. (Fig. xi.) 



Both of the tangential walls of the epidermal cells are thicker 

 than the radial and end walls, the outer one being slightly cuti- 

 nized. The stomata are subtended by small guard cells and are 

 continuous with substomatal cavities in the chlorenchyma. The 

 chlorenchyma consists of a compact subepidermal layer and several 

 adjacent layers of spongy cells which are smaller than the non- 

 chlorophyllose parenchyma centrad to this zone. 



The phloem of each collateral bundle is capped by an arc of 

 pericyclic fibers. The sectors of fibers which subtend the larger 

 bundles are more fully developed than those outside the smaller 

 ones; and, together, they form a discontinuous cylinder of mechan- 

 ical tissue. Adjacent bundles are separated by medullary rays of 

 thin-walled parenchyma but later in ontogeny the walls thicken. 

 In some instances, an interfascicular cambium may form across the 

 ray and produce several radial rows of non-vascular tissue. (Fig. 

 153.) Except for the connective tissue which forms a thickened 

 sheath around the xylem portion of each collateral bundle, the 

 pith consists of parenchymatous cells; and the stem is commonly 

 hollow at maturity owing to the disintegration of the central cells. 



The primary xylem consists of annular and spiral elements that 

 are separated from each other by thin-walled parenchyma. The 



