408 



STRUCTURE OF DICOTYLEDONOUS STEM. 



interior, whence the term Endogenous was given to this type of 

 stem- structure. From the fact just stated, however, regarding 

 the course of the Fibro -Vascular bundles, it is obvious that such a 

 doctrine cannot be any longer admitted ; for those which are most 

 recently formed only pass into the centre of the Stem during the 

 higher part of their course, and usually make their way again to 

 its exterior at no great distance below ; and thus the lower and 

 older portions of a Palm-stem really do receive very little aug- 

 mentation in diameter, while a rapid elongation is taking place at 

 its summit. In fact, the dense unyielding nature of the fabric 

 which is formed by the interlacement of the Fibro-vascular bundles 

 at or near the surface of the trunk, would prevent any considerable 

 augmentation by expanding pressure from within. 



300. In the Stems of Dicotyledonous Phanerogamia, on the other 

 hand, we find a method of arrangement of the several parts, which 



must be regarded as the highest 

 form of the development of the 

 Axis, being that in which the 

 greatest differentiation exists. 

 A distinct division is always 

 seen in a Transverse Section 

 between the three concentric 

 area? of the Pith, the Wood, and 

 the Bark; the first (a) being 

 central and the last (b) peri- 

 pheral, and these having the 

 Wood interposed between them, 

 its circle being made up of wedge 

 shaped bundles (rf,cZ),kept apart 

 by the bands (c, c) that pass be- 

 tween the Pith and the Bark. 

 The Pith (Plate xn., fig. 1, a) 

 is almost invariably composed, 

 of Cellular tissue only, which 

 usually presents (in transverse 

 section) a hexagonal areolation. 

 When newly formed it has a 

 greenish hue, and its cells are filled with fluid ; but it gradually 

 dries-up and loses its colour ; and not unfrequently its component 

 Cells are torn apart by the rapid growth of their envelope, 

 so that irregular cavities are found in it ; or, if the Stem should 

 increase with extreme rapidity, it becomes hollow, the Pith being 

 reduced to fragments, which are found adhering to its interior 

 wall. The Pith is immediately surrounded by a delicate Membrane 

 consisting almost entirely of Spiral Vessels, which is termed the 

 Medullar}/ Sheath. 



301. The Woody portion of the Stem (Plate xn., fig. 1, b, b) is 

 made up of Woody Fibres, usually with the addition of Ducts of 



Diagram of the first formation of 

 an Exogenous Stem ; — a, Pith ; b b, 

 Bark ; c c, plates of Cellular Tissue 

 (Medullary Rays) left between the 

 Woody bundles d d. 



