AND THEIR BEARING ON STELAR THEORIES FOR THE FERNS. 
717 
are present, and one of these (A) helps to define the base of the third leaf-trace (X). 
The departure of the third leaf-trace is shown in fig. 3. The steps taken in the 
liberation of the second leaf-trace are here repeated ; but opposite the leaf-trace 
not only is the stelar phloem interrupted, but the xylem is greatly reduced. The 
chief change of stelar structure at this level is due, however, to a well-defined core 
of parenchyma within the xylem (M). The sections preceding the one now figured 
show this pith to be a mass of xylem-parenchyma more extensive and continuous 
than the masses of similar nature already noted at lower levels. It is a purely 
intraxylic tissue in origin as in location. Within it is a group of tracheides (T) 
which are linked at higher and lower levels to the general xylem mass. The 
departure of the third leaf-trace is thus protostelic, and pith has been formed within 
the wood itself. At 0 a parenchymatous connection links the pith and pericycle. 
In fig. 4 the departure of the fourth leaf-trace is shown. The stelar enclodermis 
remains an uninterrupted histological barrier between the stele and cortex. Both 
stelar phloem and xylem are discontinuous opposite the leaf-trace, and pericycle and 
pith are in open parenchymatous connection through the intrastelar gap thus 
formed (0). The closure of this gap is secured in the immediately succeeding 
sections, and in the internode which follows the parenchymatous pith is increased 
and is still traversed by groups of tracheides (T, fig. 5). In fig. 6 is shown the 
stelar structure immediately above the point of liberation of the fifth leaf-trace. 
During departure of this trace the stelar endodermis is maintained unbroken. At 
the level figured the pith is composed entirely of thin-walled parenchyma (M), 
though in the immediately preceding sections it contains scattered tracheides. A 
parenchymatous gap in the cylinder of xylem and phloem places the pith and 
pericycle in direct histological connection, but at a higher level this gap is closed. 
The transition thus, traced from solid protostely without intrastelar gaps to 
protostely with a purely parenchymatous pith and intrastelar gaps is shown in the 
three series of sections of sporeling plants in G Wynne- Vaughan’s collection. In 
them the isolation of stele from cortex is maintained complete, for until stele and 
cortex are merged in the procambial mass the stelar endodermis is clearly seen as 
an unbroken ring around the pericycle. But whatever interpretation may be put 
upon these stelar changes in the light of adult structure and lateral comparison, 
in the individual sporelings referred to the following facts are clear : — 
(i) The stelar endodermis is an unbroken cylinder throughout, and no lateral 
connection between cortex and pith has been observed. 
(ii) The pith is of intrastelar origin, arising in the ontogeny by gradual increase 
of wood-parenchyma at the core of the stele, with an accompanying 
decrease of central tracheides. This intraxylic readjustment can be 
explained most reasonably as due to change of destination of procambial 
elements as the growing point advanced. 
