FIBRO VASCULAR TISSUES: WOOD 23 



known as wood rays, are a marked feature of its organization. The 

 primary wood consists typically of two regions: one composed of 

 elements with ringed and spiral thickenings and capable of elonga- 

 tion in accordance with the growth of the organs; the other con- 

 stituted of tracheary cells thickened in a reticulate, scalariform, or 

 pitted fashion and, as a consequence, incapable of extension to 

 meet the needs of lengthening parts. The former region of the 

 primary wood is designated the protoxylem and the latter the 

 metaxylem. The order of development of the protoxylem and 

 metaxylem differs significantly in different groups of plants. In 

 the very oldest forms the progress of differentiation is entirely 

 toward the center (lycopods and their allies) . In the ferns and the 

 lower gymnosperms the sequence is first central or centripetal 

 and later peripheral or centrifugal, with the result that the protoxy- 

 lem occupies a median position. In the higher gymnosperms and 

 in the angiosperms the inward order of development has nearly or 

 quite disappeared, and, as a consequence, the protoxylem lies on 

 the inside of the metaxylem, which is formed characteristically 

 outward or centrifugally. Technically these three types of organi- 

 zation of the primary wood are designated exarch, mesarch, and 

 endarch. Finally, it may be recalled that the primary wood of 

 the root of all vascular plants has the exarch organization of the 

 older types of stems and exemplifies the fact that the root in this 

 respect as in so many others (to be shown in the sequel) is the most 

 conservative of plant organs. 



