516 



PHYSIOLOGY. 



Fig. 564. 



The fibre-vascular bundles differ in their mode of growth in 

 different Classes of Plants, which, in consequence of this, exhibit 

 considerable difference in the structure of their mature stems. 

 The simplest form is absolutely without vessels, as in Mosses 

 and some simple aquatic Phanerogamia (Potamogetori), where the 

 fibro- vascular tissue is composed simply of cords of prosenchyma 

 traversing the cellular tissue. 



Complete bundles, however, possess several elements arranged in 

 definite order ; these belong to the wood-region, the cambium- 

 region, and the liber-region. The wood-region, which lies next the 

 centre of the stem, is composed of short-celled prosenchyma inter- 

 mingled with spiral and other vessels (and in Dicotyledons pitted 

 ducts) ; the cambium-region is composed of prosenchyma in a 

 nascent condition. The increase of such stems depends on the 

 development of new cells in 

 this region. The liber-region 

 is composed of very long pro- 

 senchymatous tissue (usually 

 in the condition of isolated 

 bundles of thin laminae con- 

 nected by cellular tissue in 

 the Dicotyledons). In the 

 Monocotyledons the region is 

 converted into vasa propria 

 (fig. 564). 



In the Higher Crypt ogamia 

 there is no dermatogen, but 

 only periblem surrounding the 

 plerorne ; the bundles are closed, t 

 and do not alter in their con- r 

 dition when once formed, and 

 they anastomose with those 

 that succeed them in succes- 

 sive internodes of the stem, 

 so that the fibro-vascular 

 structure appears continuous. 



In the Monocotyledons 

 there is but little periblem ; 

 the bundles are also closed, 

 are formed by degrees, a cam- 

 bium-region being formed from 

 the plerome and occupying the 

 central part at first ; but after 

 a time this is wholly resolved 

 into wood, liber, and vasa propria. 



Monocotyledonous fibro- vascular bundle (from the 

 spadix of Phoenix dactylifera). A. Transverse 

 section. B. Vertical section : , parenchyma in 

 which the bundles lie ; w, wood-cells ; s v, spiral 

 vessels; d, reticulated ducts (from w to d are 

 included in the woody portion of the bundle 

 or xylein) ; v p, vasa propria ; I, liber-cells (from 

 v p to I are included in the phloem or liber part 

 01 the bundle). Magn. 100 diam. 



These bundles remain isolated 



