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RELATIONSHIPS. 



about and between the bases of the segments of the inclosing wood, this orientation 

 marking the origin of minor and large or mesh Medullary rays. Encephalartos and 

 Macrozamia afford, however, an interesting exception. In certain, and possibly 

 all the species of these genera, a separate anastomosing system of cauline bundles 

 arises in the older portions of the pith and traverses its length and breadth in 

 almost every direction. These pith bundles lack protoxylem. Their phloem is 

 always directed toward an accompanying gum canal, and where such a canal leads 

 out into the cortex the bundle joius the normal wood zone. 



Vet further vascular struc- 

 tures that may be inclosed 

 within the pith are connected 

 with fructification. In Dion, 

 Stangcria, Ceratozamia, Zamia, 

 etc., the pith of trunks which 

 have fructified contains a great 

 number of regularly disposed 

 vascular series, each of which 

 consists in a cylinder of bundles 

 leading out into a peduncle, 

 situated laterally on the stem 

 among the old leaf bases. This 

 structure owes its origin and 

 position to the following fact : 

 A given peduncle originally 

 formed the end of the trunk 

 and contained the apical meri- 

 stem, which, after destruction 

 by the ripening of the strobilus, 

 was again produced as a sym- 

 podial branch from the pedun- 

 cular base, the latter being pressed to one side by continued growth and increase 

 of trunk diameter. It is in this way that a succession of peduncular cylindrically 

 arranged series of bundles comes to be embedded in the pith of an old stem. As 

 these series must be thrust to the one side or the other when the apical stem 

 branching occurs, they come as growth proceeds to be more or less nearly tangen- 

 tial to some one point of the vascular cylinder inclosing the pith. 



Fig. 109. — (?) Ceratozamia mexicana. X iV A cycad vege- 

 tatively much like the Zamias shown in preceding figures, but 

 with markedly more persistent armor. 



The Vascular Zone. 



Brongniart discovered the gymnospermous character of the wood of cycads 

 in 1829 (14), but he did not discern that a true phloem region follows the xylem. 

 Two types of vascular zonal development occur in the existing cycads. In (<?), 

 the simple and characteristic or monoxylic form, there is but a single xylem and 

 phloem region ; in (£), the polyxylic trunk, in addition to the normal bundle zone, 

 anomalous series form successively. 



