20 



THE ANATOMY OF WOODY PLANTS 



generally from the exterior toward the center, and the metaxylem, 

 as a consequence, is more axial or central in position than the 

 protoxylem. This mode of development of the primary wood is 

 characteristic of the most ancient plants, the lycopods and their 

 allies, and is likewise universally present in the most conservative 

 organ of all plants, the root. When the primary woody tissues 

 develop from the outer region inward, as indicated above, they are 

 said to be exarch. The situation just described will become more 

 apparent by reference to Fig. i6b, which shows i6a in an immature 



FIG. 16. a, transverse section of the bundle of an old root of Osmunda cinna- 

 momea, showing primary wood complete; b, younger condition of the same with 

 central region (metaxylem) of bundle still immature and containing protoplasm. 



condition. The outer regions of the oval mass of xylem are alone 

 developed, the center being still occupied with thin cells filled with 

 protoplasm. 



In the stem organs of the ferns and lower gymnosperms a some- 

 what different mode of development of the primary wood is char- 

 acteristically present. This may be illustrated by reference to one 

 of the fibro vascular strands of the bracken fern, Pteris aquilina. 

 The smallest elements of the wood are situated in the woody tissue 

 constituting the center of the bundle. As in the case of Lycopodium 

 and its allies, the smaller first-formed cells belong to the protoxylem. 

 The situation which presents itself in the later development of the 

 woody strand of ferns, however, is usually quite different from that 

 found in the lycopod series. In the ferns the tissues of the primary 

 metaxylem, instead of lying entirely toward the center of the organ 



