378 



STRUCTURE OF PHANEROGAMIC PLANTS. 



formed bundles descend in the interior of the stem through its 

 entire length, until they reach the roots ; and as the successive 

 development of leaves involves a successive development of new 

 bundles, the stem was imagined to be continually receiving addi- 

 tions to its 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 augmentation in diameter, while a rapid elon- 

 gation is taking place at its summit. In fact, the dense unyield- 

 ing 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 such augmentation by expanding pressure 

 from within. 



237. In the stems of Dicotyledonous Phanerogamia, we find a 

 method of arrangement of the several parts, which must be re- 

 garded as the highest form of the de- 

 velopment 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 arese of the pith, the wood, 

 and the bark ; the first (a) being central 

 and the last (b) peripheral, and these 

 having the wood interposed between 

 them, its circle being made up of wedge- 

 shaped bundles (d, d), kept apart by the 

 bands (c, c) that pass between the pith 

 and the bark. The Pith (Fig. 165, a) is 



c c, plates of ceiiuiar tissue (medui- almost in variably composed of cellular 



betweeu the W dy tissue onlv > wllich usually presents (in 

 transverse section) a hexagonal areola- 

 tion. When newly formed, it has a greenish hue, and its cells 

 are filled with fluid ; but it gradually dries up and loses its color ; 

 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, and termed the " medullary sheath." 



238. The Woody portion of the stem (Fig. 165, b, b) is made 

 up of woody fibres, usually with the addition of ducts of various 

 kinds ; these, however, are absent in one large group, the Coni- 



Diagram of the first formation of an 

 Exogenous stem; a, pith; b 6, hark ; 



