FIBRO VASCULAR TISSUES: WOOD 



length. Elements in the aggregation belonging to this category 

 are known as the metaxylem or, more specifically, as the primary 

 metaxylem. 



In the preceding paragraph, for the sake of convenience it has 

 been assumed that the order of development of the primary wood 

 is always in the same direction. As a matter of fact the time and 

 the order of appear- 

 ance of the elements 

 in this, from the 

 evolutionary stand- 

 point, highly signifi- 

 cant tissue vary 

 within certain im- 

 portant limits. We 

 may first consider 

 the most ancient 

 order of seriation of 

 the constituents of 

 the primary wood- 

 that found in the 

 stems of the most 

 antique plants and 

 in the roots of all 

 vascular organisms 

 from the lowest to 

 the highest. Fig. 15 

 illustrates the organization of the wood in a stem of the common 

 club moss, Lycopodium. The tissues of the xylem constitute a sort 

 of star, the points of which are occupied by the small-sized elements 

 of the protoxylem. As the rays of the star broaden inwardly, there is 

 a transition from protoxylem to metaxylem. The situation becomes 

 more clear by reference to transverse sections of the root in a fern 

 shown in Fig. i6a. Spiral sculpture marks the small elements on 

 the outside, while toward the center of the organ the typical sculp- 

 ture of the metaxylem becomes more and more conspicuous. In 

 the club mosses and their allies, as well as in all roots, the seriation 

 in the development of the elements of the primary wood is very 



FIG. 15. Transverse section of the upright stem of 

 Lycopodium clavatiim, showing centripetal or centrad 

 development of the primary wood; the smaller elements 

 represent the protoxylem. 



