I76 PART II. VEGETABLE HISTOLOGY. 



ever, no endodermis is present. Outside of the endodermis and 

 immediately in contact with it there may also be one or more 

 thicknesses of thick-walled cells, often fibrous in their character, 

 constituting a strengthening layer. 



According to the relative arrangement of the xylem and 

 phloem masses, three kinds of fibro-vascular bundles are distin- 

 guished, the collateral, the concentric , and the radial. 



(a) The Collateral Bundle is characterized by having the 

 xylem and phloem masses arranged side by side, and usually in 

 such a manner that the former faces interiorly, and the latter 

 exteriorly, as regards the organ that bears it. In leaves, the 

 xylem faces the upper or ventral surface, while the phloem faces 

 the lower or dorsal surface. In Fig. 436,/ is the phloem, and x 

 the xylem. Bundles of this kind are characteristic of the stems 

 and foliage leaves of nearly all flowering-plants, but they seldom 

 occur in roots. 



Two varieties are distinguished, the ordinary form, in which 

 there is one mass each of xylem and phloem, and the bi-collateral 

 bundle, in which there is one mass of xylem between two of 

 phloem. The latter variety is found only in the stems of mem- 

 bers of the gourd family, and in a few other plants. 



Some collateral bundles continue to increase in thickness 

 during the life of the organ which contains them, and the growing 

 layer is located at the junction of the xylem with the phloem. 

 Such bundles are called open bundles, while those which soon 

 cease to grow or form new tissues are called closed bundles. 

 The former are characteristic of the stems of woody Dicotyle- 

 dons. In this case, the bundles are radially arranged with the 

 phloem masses pointing outward, and the growing layer is con- 

 tinued across from bundle to bundle, constituting a continuous 

 zone, which forms the line of junction of the wood with the bark. 

 This is called the cambium zone. The cambium of a bundle is 

 shown in Fig. 436, c. The bundles found in the stems of most 

 Monocotyledons are of the closed collateral variety. 



(b) The Concentric Bundle. This differs from the last in 

 having one of the two elements of the bundle (xylem or phloem) 

 placed centrally and surrounded by the other, In the concentric 

 bundles of most Ferns and Club-mosses the xylem tissues are 

 surrounded by phloem, while in the outer series of fibro-vascular 



