412 GYMNOSPERMS 



cause the mode of origin is difTcrent. Physiologically, the com- 

 panion cells are the same in both groups, but there is no phylogenctic 

 connection. 



The root. — The root of Gnetum gncmon is diarch, with an extremely 

 scanty amount of primary xylem, only a single row of cells with 

 occasionally a couple of cells in the protoxylem lying side by side 

 (fig. 384). The vessels are of various sizes and are much larger than 

 those of the stem, with several times as great a diameter, as may be 

 seen by comparing figs. 383 and 385, which are both from Gnclum 

 gncmon, and drawn to the same scale. Vessels of lianas are likely to 

 be larger. The phloem, as in the stem, consists of very uniform 

 cells and, just beyond it, are numerous libers (fig. 385). The rays are 

 broad and thin walled, and are packed with starch, in striking con- 

 trast with those of the stem, which are thick walled and pitted. The 

 difference in structure accounts for the fact that the wood of the 

 stem is very hard, while that of the root is very soft. 



The tracheids have uniseriate bordered pits, with conspicuous 

 bars of Sanio (fig. 386) . The pits of the vessels are smaller and multi- 

 seriate, with few bars of Sanio, or even none at all. 



The breaking of the end walls of the large cells to form the con- 

 tinuous lumen of the vessels is easily seen in the root (fig. 387). At 

 first, the border of the large perforation may be ragged, but it soon 

 becomes smooth, and sometimes the passage may be even more 

 complete than that shown in the figure. 



On the whole, Gnetum, in its outward appearance and in its inter- 

 nal structure, has almost reached the angiosperm level; and, since it 

 has no record as a fossil, the temptation to connect it in some way 

 with the angiosperms has often been irresistible. 



THE SPOROPHYTE — REPRODUCTIVE 



Like the other two genera, Gnetum is dioecious; but also, like them, 

 it shows very clearly that a condition of complete dioecism has not 

 yet been attained. 



The slaminatc slrohiliis. — ^The staminate strobilus is a long slender 

 axis, bearing numerous decussate pairs of bracts, which are con- 

 crescent throughout practically their whole length, making them 



