THE HISTOLOGY OF THE EPHEDRE^. 207 



The leaf-trace, composed of two bundles, passes down the stem vertically through 

 two internodes. At the third node from that of their entrance to the stem, the bundles 

 fuse right and left with the corresponding bundles of the traces emerging at this node 

 (Plate II., fig. 13). Thus the first internode from an apical bud which has the vascular 

 tissue developed shows in cross section four bundles, while in the second and subsequent 

 internodes eight bundles are seen, two pairs of large bundles forming the traces of the 

 leaves at the second node above, alternating with two pairs of small traces supplying 

 the leaves at the first node above. A similar system is found in those species which 

 have three leaves at each node ; the extra trace pairs in this case bring the number of 

 bundles in each internode up to twelve (Plate II., fig. 15). In the internode of species 

 whose phyllotaxis changes from two to three, the internode succeeding the node at 

 which the alterations occur shows ten bundles. 



Immediately above each node occurs a region of two or three layers of compressed 

 living cells with thick cellulose walls forming a dehiscence layer (Plate III., fig. 16). 

 This layer, with in some cases a small area of meristematic tissue in the node itself, 

 arises from the remains of the meristematic tissue from which elongation of the internode 

 took place (Plate III., fig. 17). The layer partially cuts through the vascular tissues, 

 the connection of the xylem elements being kept up by means of short reticulate 

 tracheids. In the cortex the layer also appears, and there is a slight ring-like con- 

 striction of the surface of the stem corresponding to it in position. The epidermal 

 cells over the constriction are nearly isodiametric. OwiDg to the stoppage of the 

 stereom systems on approaching this region, the internode is very brittle and readily 

 breaks across at this point. While passing through the area of meristematic tissue the 

 vascular bundles lose their accompanying lateral lignified flanges. 



At the node the stelar elements are concentrated into two vascular crescents, 

 through the intercalation of linking tracheids. Spaces are left between the crescents 

 corresponding with the position of the leaves at the node (Plate III., fig. 18). Each 

 crescent is composed of the trace bundles from the leaf at the node above, flanked by 

 a trace bundle from each leaf at the second node above (see Plate II., fig. 13). The 

 leaf-traces of the leaves at the node in question link on to the crescent before pass- 

 ing out to the leaves at the node. Just below the emergence of these leaf-traces the 

 vascular supply for the axillary bud originates as a twig from each leaf-trace (see fig. 13). 



Lateral Branch. — Each of the bud- traces bifurcates to form the leaf- traces of the 

 first pair of leaves on the side branch.. At the first node of the lateral branch 

 vascular crescents are formed. The traces from the leaves at the two next higher 

 nodes unite in pairs during their course in the second internode of the branch. 

 Hence, apparently only two instead of the normal four bundles take part in the forma- 

 tion of each vascular crescent at the first node (Plate III., fig. 19). 



Accessory Bundles (Strasburger's "complementary bundles " (7) ). — Originating 

 from the centre of each vascular crescent, an accessory bundle accompanies each leaf- 

 trace pair in the upper half of its course (Plate II., fig. 14). So far, only in one instance 



