FIBRO VASCULAR TISSUES: WOOD 



21 



in the exarch condition featured in the lowest vascular plants, 

 characteristically surround the first-formed ringed and spiral ele- 

 ments (the protoxylem). This situation so frequently presented 

 by the ferns and lower gymnosperms is designated as mesarch. 

 The longitudinal topography of the bundle in this type is shown in 

 Fig. 17. 



Fig. 13, described at the outset, pictures the relative position of 

 the constituents of the primary wood in the stem of the higher seed 

 plants, the Gnetales, the Con- 

 iferales, and the angiosperms. 

 In this case, since the seria- 

 tion of the successive elements 

 is always outward from an 

 internal starting-point, the 

 primary wood is known as 

 endarch. This condition is 

 the typical one for all the 

 higher plants, and no form 

 characterized by it can, with- 

 out the clearest evidence, be 

 regarded as low in the scale 



of plants. Although the endarch condition is a feature of the 

 development of the stem and generally of the leaf in the highest 

 plants, it is important to emphasize at this stage that the root, 

 even in the most advanced organisms, betrays its extreme con- 

 servatism by adhering in its primary structures to the mode of 

 development and seriation of the elements characteristic of the 

 most ancient known plants, the lycopods and their allies. Fig. 18 

 reveals the organization of a young root of the American larch. 

 We may disregard in the present connection all but the central 

 tissues of the root. Right and left can be distinguished two 

 spaces, the resin canals. Inside of each of these two secretory 

 cavities lies a cluster of protoxylem, distinguishable by the small 

 size of its elements. Toward the center from each aggrega- 

 tion of protoxylem extends the metaxylem, which in the young 

 condition represented in the figure has not yet become joined 

 in the center. 



_ 



FIG. 17. Longitudinal section of a 

 bundle from the rootstock of Pteris aquilina, 

 showing the elements of the protoxylem in 

 the center of the wood. 



