PHANEROGAMIA : AXIAL ORGANS. 



247 



one another. From this cause, as well as from a more or less extended 

 vertical course of the vascular bundles before they form the arc, the ex- 

 ternal part of the stem is composed of a thicker cylinder of vascular 

 bundles (fig. 160. c c), whilst the inner portion, composed only of arcs, 

 becoming more and more isolated toward the centre, and cellular tissue 

 increasing in quantity in the inverse proportion, appears much looser. 

 However simple we may consider the course of the vascular bundles in 

 the Monocotyledons in judging of them according to H. Mohl's re- 

 searches *, it is, in fact, but seldom so ; nevertheless, H. Mohl's repre- 

 sentation affords the simplest and clearest delineation, 

 and gives the type from which all the analogous struc- 

 tures must be deduced. Occasionally, however, the 

 course actually appears as direct as that indicated by 

 the diagram, as, for instance, in the subterranean stem 

 , of Iris chinensis (fig. 161.). The separate vascular 

 bundles, especially so far as they form the arc, by no 

 means always run in one and the same vertical plane, 

 their emergence deviating frequently about 50 and 



more, of the circumference of the stem, laterally, from 

 the vertical of their starting point, as may be easily ob- 

 served, for instance, in Yucca gloriosa. The Xantho- 

 rhcea australis (fig. 162.) appears to me to differ most 

 strikingly from the simple type of the stem. Here the 

 fascicles of the vascular bundles, emerging into the 

 leaves (e, e, e\ evidently have a threefold origin from 

 three different zones of the stem (aa, bb, cc). Quite 

 in the interior another plexus of vascular bundles 



* De Palmarum structura. 



161 Iris chinensis, natural size. Longitudinal section of the subterranean stem. 

 a a, Cortical layer, b U b", Three leaf cicatrices, b" b"" b v , Remains of three leaves 

 which have been cut off, with the vascular bundles running to all six leaves, c, Youngest 

 leaf, still in the bud stage: to the right may be seen the terminal bud as a scarcely 

 perceptible elevation, at the right side of which the next leaf will be formed. 



16 ' Xanihorhoca australis, natural size. Longitudinal section through half the di- 

 ameter of a piece of stem (.r ). The arrow indicates the direction from within 

 outwards. The course of the separate vascular bundles has been carefully exposed by 

 artificial preparation, a, b, c, The three regions of tiie external part of the stem, which 

 give off vascular bundles for the cords (e e e) running into the leaves, rf, The inner 

 lax part of the stem, with very irregularly inclined vascular bundles, whose course 



K 1 



