ORGANIZATION: PLANT TISSUES. 



361 



perennial plants continue to grow in diameter each year. The cambium 

 in the open bundle forms new tissue each spring and summer, thus adding 

 to the phloem on the outside and the xylem on the inside. In the spring 

 and early summer the large vessels in the xylem predominate, while in 

 late summer wood fibers and small vessels predominate and this part of 

 the wood is firmer. Since the vascular bundles in the stem form a circle in 

 the cylinder, this difference in the size of the spring and late summer wood 

 produces the "annual" rings, so evident in the cross-section of a tree trunk. 

 Branches originate at the surface involving epidermis, cortex, and the 

 bundles. 



In monocotyledonous plants (corn, palm, etc.) the bundles are not regu- 

 larly arranged to form a hollow cylinder, but are irregularly situated through 

 the stele. There is no meristem, or cambium, left between the xylem and 

 phloem portions of the bundle and the bundle is thus closed, (as in fig. 60), 

 since it all passes over into permanent tissue. In most monocotyledons 

 there is, therefore, practically no annual increase in diameter of the stem. 



711. Ferns. In the ferns and most of the Pteridophytes an apical meri- 

 stem tissue is wanting, its place being taken 



by a single apical cell from the several 



sides of which cells are successively cut 



off, though Isoetes and many species of 



Lycopodium have an apical meristem 



group. In most of the Pteridophytes also 



the bundles are concentric instead of col- 



lateral. Fig. 418 represents one of the 



bundles from the stem of the polypody 



fern. The xylem is in the center, this 



surrounded by the phloem, the phloem by flff^jS^^SSSSSuA 



the phloem sheath, and this in turn by sclerenchyma; a, thin - walled 



. . sclerenchyma; par, parenchyma. 



the endodermis, giving a concentric ar- 



rangement of the component tissues. A cross-section of the stem (fig. 

 419") shows two large areas of sclerenchyma, which gives the chief mechan- 

 ical support, the bundles being comparatively weak. 



712. Origin of root tissues. A similar apical meristem exists in roots, 

 but there is in addition a fourth region of formative tissue in front of the 

 meristem called calyptrogen (fig. 420). This gives rise to the "root cap" 

 which serves to protect the meristem. The vascular cylinder in roots is 

 very different from that of the stem. There is a solid central cylinder in 

 which the groups of xylem radiate from the center and groups of phloem 

 alternate with them but do not extend so near the center (fig. 421). As the 

 root ages, changes take place which obscure this arrangement more or 

 less. Branches of the roots arise from the central cylinder. In fern 

 roots the apical meristem is replaced by a single four-sided (tetrahedral) 



Fig- 419- 



