320 
Hill and de Fraine.—On the 
The mesophyll is homogeneous and consists of normal parenchy¬ 
matous cells ; the intercellular space system is not highly developed. 
Each cotyledon contains two vascular bundles which are collateral 
in structure and do not exhibit any vascular rearrangement within the 
seed-leaves. Transfusion tracheides, so far as has been seen, are absent, 
and no fibrous elements, abutting directly on to the soft bast, such as occur 
in so many species of Pinus, &c., have been observed (PI. XXII, Fig. i). 
E. altissima , however, does possess a number of fibrous elements, with 
thickened, but apparently unlignified walls, scattered throughout the meso¬ 
phyll of the seed-leaves and in the cortical ground-tissue of the hypocotyl. 
The orientation of the bundles of a cotyledon is peculiar ; they are 
arranged in such a manner that the plane of the bundles is at right angles, 
or nearly so, to the dorso-ventral plane of the leaf; the protoxylem of one 
bundle thus is directed towards the same tissue of the strand on the other 
side (Diagram I, Fig. i). 
In all cases a well-marked cotyledonary tube is formed (Diagram i, 
Fig. 2 ). 
Transition. 
E. distachya. The orientation of the bundles of the cotyledons just 
remarked upon obtains throughout the whole length of the seed-leaves, 
and in this condition they enter the hypocotyledonary axis ; thus there are 
four bundles in the hypocotyl arranged in two well-defined pairs (Diagram 1 , 
Fig. 3 ). During the downward passage the two strands from each cotyle¬ 
don approach one another and, concurrently, a rearrangement of the elements 
of the wood takes place, which leads finally to the fusion of the protoxylem 
elements (Diagram 1 , Fig. 4 ). The approachment continues so that the 
xylem-masses of the bundles of each pair come into continuity, with their 
protoxylems in an exarch position (Diagram 1 , Fig. 5 ). The four groups 
of phloem elements still retain their identity, and continue so to do through 
the greater length of the root of young seedlings (Diagram 1 , Fig. 6 ) ; 
indeed, we have seen no absolutely certain case in which the phloem, which, 
in this genus, generally is very poorly defined when viewed in transverse 
section, forms two well-marked strands as would be expected. This fact is 
curious, and may be compared with the similar state of affairs which occurs 
in certain Cactaceae . 1 
The transition-phenomena in E. campylopodia, E. fragilis , and 
E. altissima are precisely similar to those obtaining in E. distachya . 
Attention may now be drawn to a rather interesting feature occurring 
in the hypocotyl of all the seedlings of the species of Ephedra examined. 
At a level just below the cotyledonary node a shallow band of inter¬ 
fascicular cambium occurs which joins the fascicular cambium of the 
opposing bundles of each cotyledonary pair (Diagram 1 , Fig. 3 ). This 
1 See de Fraine, On the seedling structure of Cactaceae (Ann. Bot., xxiv, Jan., 1910). 
