FIBRO-VASCULAR BUNDLES. 57 



thus the xylem and phloem bundles alternate with each 

 other in one ring, and are not collateral as in stems. In the 

 xylem bundles the spiral vessels, the oldest portions, lie 

 nearest the periphery of the root, the opposite arrangement 

 to that met with in the stem. In open bundles the cam- 

 bium ring is outside the primary xylem and inside the 

 primary phloem. The tissue immediately surrounding the 

 fibro-vascular cylinder is called pericambium ; the secondary 

 roots originate in this pericambium, usually opposite to the 

 xylem bundles, and during their development push through 

 the cortex of the mother-root ; for this reason roots are said 

 to be endogenous in origin, and in this respect differ from 

 branches. 



Pericambium is absent from the roots in the species of 

 Equisetum or Horsetails. 



The appearance and gradual differentiation of the fibro- 

 vascular system in plants is to a very great extent contem- 

 poraneous with their change of habitat from an aquatic to 

 an serial mode of life. In aquatic plants of a primitive type, 

 at all events, the entire tissue consisted of parenchyma, and 

 the absence of a waterproof epidermis from every part 

 enabled every portion of the plant to absorb the required 

 amount of water, consequently no internal arrangement was 

 necessary for the transmission of this essential medium from 

 one part to another ; and as the water taken in contained in 

 solution the gases required for food and respiratory purposes, 

 special arrangements for this purpose were not required. 

 On the other hand, when plants had succeeded in establish- 

 ing themselves on land, as already explained, those parts 

 that remained in the damp ground, as roots, were the only 

 portions capable of taking in water, as in all plants above 

 mosses those portions growing in the air are protected by 



