1 74 Gibson —Contributions towards a Knowledge 
the axis are united by metaxylem before fusion with the 
branch, but they separate again almost immediately. 
There is an epidermis, hypodermis, and thick cortex, the 
last being composed of thin-walled parenchyma when young, 
but thick-walled and deeply pitted in older stems. The 
lacuna is sharply defined. The trabeculae consist of endo- 
dermal cells with or without a cluster of green parenchymatous 
creeping cells lying against the innermost cortex (PL X, Fig. 42). 
The pericycle consists of one layer of cells, as a rule, though 
two layers occur opposite the protoxylems. There is one 
layer of sieve-tubes, absent opposite the protoxylems on the 
margins of the stele, where also a few crushed protophloem- 
elements occur. Two or three layers of parenchyma separate 
the sieve-tubes from the xylem. The swollen articulation is 
due entirely to hypertrophy of the middle layers of the cortex 
in that region, and does not affect the stele. 
38. Selaginella delicatissima , A. Br. Baker’s Handbook, 
No. 34. 
The primary and secondary axes in this species are 
nearly equal, and the branching appears for that reason to be 
dichotomous, but falsely so. The leaf-traces of the two rows 
of leaves on either side fuse into a distinct stele, each lying in 
a wide lacuna. At the youngest branchings considerable varia- 
tion occurs in the method of union of the steles (PL X, Fig. 43). 
(ci) The steles of the branch and of the axis may both fuse in 
pairs, the two steles resulting from this fusion becoming the 
steles of the axis beneath. This is what generally happens at 
the first apical branching, (b) In the second type the same 
fusion in pairs occurs, but just at the points of fusion meta- 
xylem-elements are developed so that these points are brought 
into close proximity, although actual fusion does not take 
place. (. c ) The third variation shows complete fusion of these 
points of metaxylem and consequent formation of a short 
anastomotic stele, (d) Lastly, and this is a method usually 
seen in slightly older branchings, the outer stele of the chief 
axis runs distinct whilst the inner stele of the axis fuses with 
the fused pair of steles derived from the branch. In older 
