i86 



PART II. VEGETABLE HISTOLOGY. 



trate the cylinder to the same depth, some bend downward soon 

 after entering it, others pass nearly or quite to its centre. Hence 

 the irregular arrangement observed in cross-section. It is evi- 

 dent also that the bundles will be more numerous and crowded 

 toward the periphery of the cylinder. The fact that stems of 

 this type are denser exteriorly than they are in the centre, is 

 thus, in part at least, accounted for. In some plants the central 

 portion of the cylinder is destitute of bundles, and the thin- 

 walled parenchyma of this region may even disappear at an early 

 stage in the development of the 

 stem, leaving a hollow, as in 

 the stems of most Grasses. The 

 bundles attain their best devel- 

 opment in the middle portion 

 of their course. In their prog- 

 ress downward and outward 

 they become thinner and less 

 vascular ; hence, in viewing a 

 cross-section of a stem of this 

 kind, the best developed bun- 

 dles are found to be those 

 farthest interior, while the more 

 imperfect ones are the ones 

 crowded together toward the 

 outside of the cylinder. The 

 main points of structure in 

 stems of this type are illustrated in Figs. 446 to 448, inclusive. 

 Since the fibro-vascular bundles are closed, that is, do not 

 possess a meristem layer, no considerable increase in the thick- 

 ness of the stem can take place except by the formation of new 

 bundles. Accordingly, the majority of stems of this type do not 

 increase in thickness except when quite young. This is true 

 even when they live on from year to year; the stem of a Palm, 

 for example, having the same diameter when two feet high that 

 it has after reaching the height of one hundred feet. The grow- 

 ing area of the stem is confined to the apex or its immediate 

 vicinity, and does not extend downward as a meristem cylinder 

 between wood and bark, as it does in the stems of Gymnosperms 

 and Dicotyledons. In fact, in Monocotyledons no true bark is- 



Fig. 446. Transverse section of a rather 

 young stem of Smilax glauca, showing 

 arrangement of fibro-vascular bundles, a, 

 primary cortex; 6, crowded and depauperate 

 bundles, forming nearly a ring at the cir- 

 cumference of the central cylinder; c, one 

 of the interior bundles; d, ground-ti>sue. 

 Magnified 13 diameters. 



