DICOTYLEDONES 



403 



The primary root in many Dicotyledons persists as a tap-root, 

 as in the Gymnosperins, but this is by no means always the case, and 

 it may very soon be replaced by the secondary roots. 



Tissues of the Young Sporophyte. The cotyledons, like the later 

 leaves of most Dicotyledons, are reticulately veined, and into each 

 leaf passes one or more vascular bundles, forming the 

 " leaf-trace." These proceed downward from the base of 

 the leaf, and unite near the base of the hypocotyl, where 

 they pass into the vascular cylinder 

 of the primary root. Most commonly 

 there are two bundles in each leaf- 

 trace, which unite into a single one 

 within the hypocotyl; or one bundle 

 of each trace unites with one belonging 

 to the other cotyledon. In both cases a 

 section of the young hypocotyl shows 

 two bundles, in one case opposite the 

 cotyledons, in the other alternating 

 with them. When the number of bun- 

 dles in the traces is larger, some may 

 remain separate, and a section of the 

 hypocotyl (Fig. 383, B) shows a circle 

 of four or more bundles (Cucumis, 

 Ricinus). 



Primary Root. A section of the 

 young root shows a central vascular 

 cylinder, bounded by a more or less 

 well-defined endodermis. The root- 

 bundle is similar to that in other vascu- 

 lar plants, and is radial in structure. 

 The xylem is composed of (usually) 

 two or four masses, alternately along 

 different radii with as many masses of 

 phloem. This arrangement is soon 

 obliterated by the development of a 

 ring of cambium outside both xylem 

 and phloem. The presence of the cam- 

 bium enables the tap-root to grow in FIG. 380. Seedling of Ipomoea 

 thickness, as in the Gymnosperms. purpurea. 



As the seedling develops new leaves, the number of leaf-traces 

 increases, and these pass downward, uniting with those from the 

 older leaves, and becoming more numerous, until the normal number 

 is reached (Fig. 382). The bundles are, with very few exceptions, of 

 the typical collateral form, and are arranged in a single circle. The 

 development of cambium takes place early, and the secondary growth 



