Anatomy of Acrostichum aureum. 183 
now fuse, forming a three-armed structure, the lateral arms of which 
give off bundles to form the Series d of the upper side; the middle 
arm forms the large bundle with many protoxylems, which will give 
rise to the central Series c. (Text-figs. 33 and 34). 
Fig. 36. 
Fig. 36. Base of very old petiole. Series a, b and c much as in Fig. 35, but 
Series c composed of a ring of bundles. (See F of Fig. 37.) 
The presumption therefore of the essential horseshoe character 
of the complicated leaf-trace seen in the mature petiole, is entirely 
justified by the detailed examination of its origin at the node. The 
method of the elaboration of the horseshoe into its final form can thus 
be very clearly made out (Text-fig. 37). It consists essentially of 
the involution of the sides of the horseshoe, and fusion of the 
lateral portions near the free ends, while at the same time the 
whole structure becomes divided up into bundles. The process 
may be divided roughly into four stages (i.) differentiation of the 
middle portion of the horseshoe into Series a (Fig 37, A & B); (ii.) 
infolding of the sides of the horseshoe and differentiation from 
them of Series b (Fig. 37, C & D); (iii.) fusion of opposite sides 
of the horseshoe just above the free ends, and differentiation 
from these ends of Series d (Fig. 37, E); (iv.) formation of Series c 
from fused portion (Fig. 37, F). 
CL 
A 
O O Q 
'T-t^Vrr rr<^h-T 
Fig. 37. 
Fig. 37. Purely diagramatic representation of stages in formation of compli¬ 
cated leaf trace. It is represented as a continuous curve, while the short 
projecting teeth indicate the position of the protoxylem groups. Notice 
that they are on the inside of the curve. 
A & B. Curve of Series a. (Figs. 27-29.) 
C & D. Involution of sides of curve to form b. (Figs. 30-33.) 
E & F. Differentiation of Series c and Series d from ends of horse¬ 
shoe shaped trace. (Figs. 34-36.) 
