762 de Bruyn.—The Ontogenetic Development of the 
this makes the endodermis, especially in the young plants, often difficult to 
recognize. 
Pericycle. The pericycle consists of one or two rows of cells, and 
these cells may have grey contents. The pericycle of the mature plant, as 
stated by Seward and Dale, 1 consists of two or three rows of cells. 
Xylem. In the young plants the xylem is formed of scalariform 
tracheides, between which occasionally parenchymatous cells are found ; in 
older plants the parenchymatous cells are more numerous. 
Phloem. The phloem consists of small elements, many of which often 
possess nuclei. In the older plants a pith is present, the cells of which 
have sclerotic walls ; they are very much like the cells of the inner cortex 
next to the stele. 
Cortex. The cortex in the youngest stems is formed by uniform cells. 
In the older plants the outer layers of the cortex have cells with thicker 
walls than the inner ones, and in quite young plants this difference can 
already be noticed. 
The Ontogeny of the Stele. The extreme base of the stem of D. conju- 
gata was not present at all; it is therefore impossible to be certain that 
a protostele is present, although it is highly probable. The stele of the 
young stem is often of the Lindsaya type 2 (amphiphloic protostele), and 
the ventral side of the xylem-ring is frequently much thicker than the 
dorsal, as in the adult stem of Lindsaya itself. This is not, however, a con¬ 
stant feature, as we may find young stages with a more equally developed 
xylem-ring. Near the base of the stem the tissue surrounded by the xylem 
has a parenchymatous structure. It is very difficult to tell in the early 
stages whether any of the cells of the central parenchyma are sieve tubes. 
Later on undoubted phloem is found, and only then can we say with cer¬ 
tainty that the whole stele is of the Lindsaya type. A striking phenomenon 
is the appearance of cells with dark contents in the central tissue. Only 
a very few of these dark cells are formed at first. They do not form any 
regular layer, neither have they a constant position. They generally closely 
resemble the endodermal or pericyclic cells. The contents of the pericyclic 
cells are mostly not so dark as those of the endodermal ones. It is quite 
possible that the dark cells formed in the centre of the young stele should 
really be regarded as the common mother-cells of pericycle and endodermis. 
The behaviour of these dark internal cells during the first stages is 
quite interesting. As stated, a few of these cells appear in the centre of the 
stele, but in the next sections their number decreases or they disappear 
completely. That they are really absent and not merely unrecognizable 
owing to the absence of their dark contents, is shown by the fact that when 
they disappear the whole central tissue is formed by a smaller number of 
1 Seward and Dale, loc. cit., p. 494. 
2 Tansley and Lulham : On a New Type of Fern Stele, &c. Ann. of Bot., vol. xvi, 1902. 
