SPERMATOPHYTA 



371 



and later give rise to secondary xylem and phloem, thus forming a contin- 

 uous vascular cylinder, or it may produce only parenchyma between the 

 bundles, which then remain separate. In many herbs the interfascicular 

 cambium fails to develop at all, the secondary vascular tissues then 

 being produced within the bundles. 



Thus the stems of herbaceous dicotyledons illustrate various degrees 

 of reduction from the more highly organized but more primitive condition 



Fig. 316. Cro.ss section of a young stem of Indian corn {Zea mays), showing scattered 

 vascular bundles, X8. 



seen in the stems of typical woody dicotyledons. This strongly indicates 

 that the woody stem is the more ancient type from which the herbaceous 

 stem has been derived, probably in response to climatic changes. 



The stems of most monocotyledons display a characteristic type of 

 dictyostele with scattered vascular bundles (Fig. 316). With rare excep- 

 tions, a cambium is wanting, and so no secondary thickening ordinarily 

 occurs. In a few monocotyledons, such as Dracaena, Aloe, and Yucca, a 

 special kind of secondary thickening takes place. Here a cambium arises 

 in the pericycle or inner cortex and forms a cylinder of new vascular 

 tissues. 



In most monocotyledons the arrangement of the conducting tissues in 

 each vascular bundle is collateral, as in dicotyledons, but frequently it 

 is amphivasal, the xylem surrounding the phloem (Fig. 317). The young 

 stem of a monocotyledon is usually a siphonostele with collateral bundles. 

 The monocotyledons represent, in their stem structure, the final stages in 



