FIBRO- VASCULAR BUNDLES AND TYPES OF STEMS. 89 



CHAPTER XXII. 



FIBRO- VASCULAR BUNDLES AND TYPES OF STEMS. 



The nature of a fibro-vascular bundle was considered in the 

 lessons on wood and bast fibres and tracheary tissue, but the 

 various types of bundles were not discussed. These will be 

 considered now. 



According to the relative arrangement of the xylem and 

 phloem masses three kinds of fibro-vascular bundles are dis- 

 tinguished, namely, collateral,, concentric and radial. 



The collateral type is characterized by having the xylem and 

 phloem masses lying side by side with the xylem facing to- 

 wards the pith or center of the stem and the phloem towards 

 the exterior. In the veins of leaves the xylem faces the upper 

 or ventral surface, the phloem the lower or dorsal surface. Col- 

 lateral bundles are characteristic of the stems and leaves of 

 nearly all flowering plants. They seldom occur in roots. There 

 are two varieites of the collateral type : 



The ordinary bundle, containing one phloem and one xylem 

 mass, and the ^collateral bundle, in which there is one xylem 

 mass between two phloem masses or vice versa. The second 

 variety is found only in the stems of gourd plants (Cucurbi- 

 tacese) and a few others. Some collateral bundles continue to 

 increase in thickness during the life of the plant, and the grow- 

 ing layer is located at the junction of the xylem and phloem, 

 forming a cambium or meristem zone of the bundle. Such bun- 

 dles are called open bundles, while those which have no cam- 

 bium zone, and thus soon cease to grow, are called closed 

 bundles. 



The open bundles are characteristic of the stems of woody 

 dicotyls. An illustration of this kind of bundle has been seen 

 in the Geranium stem. The stems of most monocotyls contain 

 the closed collateral bundles. 



CONCENTRIC BUNDLES. These have a central xylem mass sur- 

 rounded by a phloem mass or vice versa. There is no cambium 

 zone in this type. The bundle with xylem central is character- 

 istic of nearly all ferns and club mosses. The one with phloem 

 central occurs only in stems and leaves of some monocotyls. 



RADIAL BUNDLES. In these the xylem tissues are arranged in 

 radial masses and are separated from one another by the phloem 

 masses, together with some parenchyma cells. Such bundles 

 are characteristic of the roots of all phanerogams and pterido- 

 phytes and stems of Lycopodiacese. 



