374 



BOTANY 



treelike Liliaceae, e.g. Yucca the stem ultimately forms a trunk, 

 which may in the latter increase in diameter as the plant grows 

 older, but in the Palms rarely shows any thickening after the crown 

 of leaves has reached its full size. In most Palms the elongation 

 of the stem does not begin until the crown of leaves is full grown, 

 and then the elongating trunk remains of nearly uniform diameter 

 throughout. Sometimes the stem is slender and freely branched 

 e.g. Zannichellia, Potamogeton, Asparagus. 



FIG. 346. Iris Florentine. Vascular bundle from the scape ( X 250) . A, cross-section. 

 B, longitudinal section; ph, phloem; t, tracheids; s, sieve-tube. 



THE MATURE SPOROPHYTE 

 The Stem 



The internal structure of the stem is much the same in all Mono- 

 cotyledons (Fig. 347, D). The bulk of the stem is composed of 

 parenchyma, through which are scattered the numerous collateral 

 vascular bundles, which never show the secondary thickening found 

 in the stem-bundles of the Gymnosperms and Dicotyledons. These 

 bundles are all leaf-traces, and in large stems, like those of the 

 Palms, each bundle is surrounded by a sheath of fibrous cells, which 

 act as mechanical or supporting elements, as the xylem of the 

 bundles is always slightly developed and serves only for conduction. 

 The other mechanical elements consist of hypodermal tissue, which 

 may be collenchyma or fibrous tissue. 



