CHAPTER III. ORGANS OF PLANTS. 



I8 7 



found, nor is there a true pith, nor medullary rays, nor rings of 

 growth. 



There are some exceptional Monocotyledons, however, like 

 the Yuccas and Dracaenas, whose stems do increase in diameter 

 from year to year by means of a meristem zone in the thick 

 primary cortex. In this layer new fibro-vascular bundles are 

 formed resembling those first formed in the central cylinder of 

 these and other Monocotyledons. This mode of growth will be 

 understood by reference to Fig. 449- 



Fig. 447. 



Fig. 448. 



Fig. 447. — One of the fibro-vascular bundles of Smilax glauca, enlarged to 155 diame- 

 ters, a, one of the two large ducts in the xylem; b, a smaller duct; c, phloem tissues; d, 

 one of the parenchyma cells of iji-: ground tissues. 



Fig. 448. — Diagram showfcj course of bundles longitudinally in Monocotyledon type 

 of stem. 



It should also be observed that there are a few anomylous 

 Monocotyledons that have their fibro-vascular bundles arranged 

 in a manner quite different from that which has been described. 

 In the common Yam and a few other plants the above-ground 

 stems have the bundles arranged in a circle about a central pith, as 

 in most Dicotyledons and Gymnosperms; and in some instances, 

 as in Ruppia and its allies and in some Potomogetons, there is a 

 single axile bundle which sends out branches to the leaves. 



The Dicotyledon Type. Here the course of the bundles 

 from the leaves is inward and usually obliquely downward, and, 



