504 THE STRUCTURE OF ECONOMIC PLANTS 



two bicollateral bundles. The xylem of the median bundle is 

 not endarch in its development, but consists of a single protoxylem 

 strand and two tangentially oriented metaxylem groups which lie 

 between the outer and inner phloem. 



The Cotyledons. — The slender cotyledonary petiole equals 

 or slightly exceeds the lamina in length, and its adaxial surface 

 is grooved so that it is crescentic in transection. There are four 

 bundles at its base; and in the two large median bicollateral 

 bundles, the outer phloem consists of several scattered groups of 

 cells while the inner phloem is composed of two or three strands. 

 In some instances, the xylem may be surrounded by a discontinuous 

 ring of phloem elements so that the bundle is essentially amphi- 



cribral. The two smaller 

 lateral bundles are col- 

 ■" ^^ lateral or half-amphicri- 

 bral. (Fig. 163.) The par- 

 enchymatous cells sur- 

 rounding the bundles are 

 thin-walled with large in- 

 ^"--m bu tercellular spaces. Toward 

 the abaxial surface, the 

 outer cell layers are com- 



FiG. 2.6^. Transection of cotyledonary petiole: j ,, 1 



/ *«, lateral bundle ; ^ bu, median bundle. P^Ct and COllenchymatOUS 



but contain chloroplasts. 

 The subepidermal cells of the adaxial surface are also chloro- 

 phyllose, but not so compactly arranged, and the more numerous 

 stomata open into large intercellular spaces. 



The arrangement of the petiolar bundles remains unchanged up 

 to a point slightly below the lamina, where the two median bundles 

 anastomose to form a single broad one. (Fig. 164, A.^ At the 

 base of the lamina, the two small bundles (a, a'') curve abruptly 

 outward and supply the auricles. The median bundle cuts off two 

 lateral branches (b, b'^ which immediately give rise to two small 

 laterals (c, c') that supply the basal portion of the lamina. The 

 median bundle finally forks just below the notch of the blade, one 

 branch extending into each lobe (d, d'^, and these with the branches 

 Qb, b''), supply the lobes of the cotyledon. 



The median bundle is still bicollateral at the base of the lamina, 

 but the inner (adaxial) phloem ends blindly in the mesophyll before 

 the strand forks. The other veins are collateral throughout their 



