262 STKUCTUKE OF STEMS AND ROOTS 



in size during life by the presence of a growing layer at 

 the line of union of the phloem and xylem. This layer 

 of growing cells is called cambium. Dicotyledonous stems 

 have open collateral bundles. Fig. 402. Closed bundles 

 are those which cease growing very early and have no 

 cambium or growing layer. They are called closed, per- 

 haps from the fact that there is no means by which they 

 may become larger. Stems of monocotyledons have 

 bundles of the closed collateral type. Examine with high 



403. Cross-section of root of brake (Pteris aquilina), showing 12 concentric fibro- 

 vascular bundles. The two long dark strands are composed of fibrous tissue. 



power cross -sections of menispermum stems and corn 

 stems (see Figs. 401, 402, 404), which have been stained 

 with hematoxylin. Study the tissues found in a single 

 bundle of each, with the aid of the illustrations. 



426. In concentric bundles, the xylem is centrally 

 placed in the bundle and the phloem is all around it, 

 as in club mosses and ferns (Fig. 403) ; or the phloem is 

 in the center of the bundle and the xylem surrounds it, 

 as in the underground stems of some monocotyledons, as 

 asparagus. Figs. 405, 406. To see concentric bundles: 



