THE WHITE LILY 159 



hand, the whole of the procambial strand is used up 

 to form permanent tissue. The xylem and phloem 

 meet ; no layer of cambium is left between them ; the 

 bundle is a closed one, and when once formed is quite 

 incapable of any further growth. 



This is one of the most constant distinctions between 

 the two great Classes of which we have taken the 

 Wallflower and the Lily as types. The one has open 

 bundles, capable of indefinite growth by means of 

 cambium, while the other has closed bundles with no 

 cambium, and therefore with limited growth. 



All Monocotyledons have closed vascular bundles, 

 and the great majority have no cambium at all, so 

 that the stem or root, when once developed from the 

 growing point, increases no more in thickness. There 

 are, however, a few members of the Class, such as the 

 Dragon-tree and Yucca, which have a kind of cambium, 

 and in which secondary growth in thickness conse- 

 quently takes place. In these plants the cambium 

 arises altogether outside the vascular bundles, so that 

 the growth is quite different from that in typical 

 Dicotyledons. We need not, however, concern our- 

 selves here with these exceptional cases. 



The cortex requires little description. Its cells 

 have thin cellulose walls, and in their protoplasm 

 chlorophyll granules are embedded, by means of which 

 the stem is able to do some assimilation. The leaves, 

 however, are the chief organs for this function. 



The epidermis has elongated cells, and rather 

 numerous large stomata, placed lengthways, so that 

 a transverse section cuts across both guard-cells. 



