Stele in Two Species of Dip ter is. 771 
3. Cells with dark contents, probably representing the beginning of an 
internal endodermis, appear isolated or in irregular masses in the midst of 
the central tissue (phloem or parenchyma). These cells may appear and 
disappear again several times as the stele is traced upwards, but make no 
connexion with the external endodermis at the leaf-gaps or elsewhere till 
after the departure of several traces. 
4. Regular connexion of the dark cell-strand with the outer endodermis 
at the succeeding leaf-gaps occurs after some time; eventually pith cells 
appear in the centre of the group of dark cells and a normal solenostele is 
formed. The solenostele of D , conjugata frequently retains for some time 
the inequality of thickness of its xylem on the dorsal and ventral sides 
noticed in the Lindsay a stage, but the xylem-ring eventually becomes of 
uniform thickness all round. 
5. The thickening of the xylem on the edges of the leaf-gap described 
by Seward and Dale in mature plants of D. conjugata occurs in the soleno¬ 
stele of quite young plants. 
6 . A feature of the steles of the young plants in both the species of 
Dipteris investigated is the irregularity of development in several of the 
stelar tissues. Thus the external endodermis is often irregularly arranged 
and is frequently double for part of its course. The pericycle sometimes 
contains similar dark contents. The dark cells which appear in the midst 
of the internal phloem or parenchyma in the young plants are generally 
regarded as being endodermal in nature, but they may be partly of peri- 
cyclic nature, or they may represent the common mother-cells of endodermis 
and pericycle. The development of the xylem is frequently irregular, the 
tracheides failing to develop in parts of the xylem-ring, which again becomes 
completely differentiated nearer the apex. 
Local development of xylem in advance of the general development is 
often found in the neighbourhood of the apex of the rhizome at the point 
where a root or a leaf-trace is inserted. 
7. In Dipteris conjugata the petiolar stele is always single, in the earlier 
leaf-traces a flattish arch of xylem surrounded by phloem, in the later a more 
strongly curved arch. Sometimes sclerenchyma surrounded by an endo¬ 
dermis is found in the centre of the stele of young petioles. These internal 
tissues differ very much in structure and are not formed in the same way 
in different petioles. Sometimes the internal sclerenchyma and internal 
endodermis are in connexion with the cortex and external endodermis; in 
other cases they arise and disappear quite independently. Dichotomy of 
the leaf-trace to form the main veins of the leaf takes place only at the 
upper end of the petiole. 
In D. Lobbiana the petiolar steles of the first-formed leaves resemble 
those of D. conjugata , but in later-formed leaf-traces a double structure 
may be evident in the lower part of the petiole; this becomes single again 
