

442 



FOURTH GROUP. SEED-PLANTS. 



fnv 



not show the bundles arranged in a simple ring, as in the Coniferae and Dicotyledons ; 

 but inside the peripheral zone of cortex which has no bundles there is a circular 

 surface, in which either a number of rows of bundles irregularly disposed are concen- 

 trically ranged round the central portion which is without bundles, as for instance in 

 the stems of many Grasses which subsequently become hollow ; or the bundles lie 

 scattered over the whole surface. This arrangement of the bundles, to which however 

 there are many exceptions, is due to the obliquely radial course of the leaf-trace- 

 bundles. These enter the stem in numbers side by side 

 from the broad insertions of the leaves, run obliquely 

 downwards penetrating to some depth into the stem, 

 then bend again outwards and as they descend again 

 approach the surface of the stem ; the bundles do not 

 all penetrate equally far into the stem, as Fig. 373 

 shows, some even keeping near the surface throughout 

 their course. The common bundle is usually thickest 

 and most highly developed at the bend where it has 

 reached farthest into the stem, while the extremity which 

 bends upwards into the leaf and the descending portion 

 become gradually slenderer and more simple in con- 

 struction. A transverse section of the stem passes 

 through the different leaf-trace-bundles at different 

 heights in their course and shows therefore bundles of 

 different structure and size ; a radial longitudinal section 

 through the bud or through fully developed stems with 

 short internodes (Palms, thick rhizomes, bulbs, etc.) 

 shows how the bundles which descend from the different 

 leaves, the curvatures of which lie at different heights, 

 cross each other in the radial direction, some running 

 inwards at the spot where others are already turning their 

 course outwards. All bundles descend through many 

 internodes, and finally unite in the outer part of the 

 vascular cylinder with bundles which emerge lower down 

 by applying themselves to them in a radial or oblique direction. In elongated internodes, 

 as in the stalks of grasses, the stems of such Palms as Calamus and the long scapes 

 of A Ilium, the bundles run nearly parallel to one another and to the surface; the 

 places where the bundles bend and cross one another, easily observed at the apex of 

 such stems, are to be found also in the transverse plates (nodes) which have not 

 lengthened between each pair of internodes, where they often form a net-work of 

 horizontal bundles, as may be seen very distinctly in Zea Mais. 



The course of the bundles as here described necessarily excludes such a distinction 

 of the fundamental tissue of the stem into pith and cortex, as we find in Conifers and 

 Dicotyledons; the parenchymatous fundamental tissue between the usually numerous 

 bundles is uniform in its character ; but sometimes it is divided into an outer peripheral 

 layer and an inner mass by the formation of a layer of tissue between them, with its 

 cells thickened and lignified in a peculiar manner, the strengthening zone, as in most of 

 the thicker rhizomes and in the hollow scape of Allium. 



Since the leaf-trace-bundles in the stem of the Monocotyledons are not parallel to one 

 another and are irregularly distributed on the transverse section, they are not adapted 

 to unite into a closed cylinder by the formation of interfascicular cambium, as is the 

 case in other Phanerogams, and it is in accordance with this that they have also no 

 active cambium-layer between the phloem and xylem ; in other words they are closed 

 bundles. When the growth in length of any given portion of the stem comes to an end 

 the whole of the tissue of the bundles becomes permanent tissue, and there is therefore 

 as a rule no subsequent growth in thickness; each portion of the stem when once 



FIG. 373. Diagrammatic representation 

 of the course of the vascular bundles in the 

 Palms, the leaves being supposed to alter- 

 nate in two rows and to embrace the stem. 

 The successive leaf-traces are numbered in 

 order; m the median bundle. After de 

 Bary, VergL Anatomic. 



