770 de Bruyn .— The Ontogenetic Development of the 
two xylem-groups (Fig. 18, PI. LVIII). Protoxylem is probably present. 
The phloem of the ventral side has joined that of the dorsal side and the 
endodermis extends somewhat inwards. The whole strand has the appear¬ 
ance of the beginning of a branching, but i cm. higher the strand has 
again single structure (Fig. 19). The tracheides form here an uninter¬ 
rupted arch surrounded by phloem which does not line the concavity. 
Protoxylem is again present. At the distance of 1*5 cm. from the lamina 
the strand divides into two, and near the lamina a second dichotomy occurs. 
Four strands are thus formed, as seen in Fig. 20. Protoxylem remains 
visible. The veins in the leaves are eventually collateral; they are often 
surrounded by sclerotic tissue. 
In the oldest plant the petiole has two separate strands. Only a very 
small part of the petiole was available, and in this part (3 cm. long) no 
change takes place, the two separate bundles maintaining the same distance 
apart. Each bundle is formed by a xylem arch surrounded by phloem. 
Endarch protoxylem is probably present. In the first section no tannin 
cells are found, but somewhat higher characteristic tannin sacs occur. They 
are not, however, always formed in the same tissue, for they may be next 
to the xylem in the conjunctive parenchyma, or they may occur in the 
phloem. 
The petiole described first has a single stelar structure, and branching 
takes place only quite near the lamina; in the oldest plant the leaf-traces 
go off with a double strand from the stem stele. The transition stage 
between these two types is quite distinctly formed by the second plant 
with its double xylem-strand at the base, and again single structure 
afterwards. 
In conclusion I want to express my thanks to Mr. Tansley for supply¬ 
ing me with the material and for all his kind help and advice throughout my 
work. The photographs were taken by Mr. Mangham, to whom I should 
also like to express my gratitude. 
Summary. 
1. The evolution of the stele in the young plants of Dipteris conjugata 
and D. Lobbicinct appears to be essentially similar. In D. conjugata the 
earliest stages were not available, but there is little doubt that a proto- 
stele would be found at the extreme base of the young stem, as it is in 
D. Lobbiana. 
2. The central tissue formed in the middle of the xylem appears at 
first to consist largely, perhaps sometimes exclusively, of parenchyma. 
Later, sieve tubes appear and a typical Lindsay a stage is reached. In 
D. conjugata the ventral side of the xylem-ring is distinctly thicker than 
the dorsal. 
