724 
DR JOHN M‘LEAN THOMPSON ON NEW STELA R FACTS, 
extrastelar, but no longer communicates with the peripheral cortex.” For the 
establishment of this condition he assumed that pocketing had occurred in the 
ancestry of Platyzomco, but that the confluent pockets by which the medulla arose 
have been “ completely cut off from the outside cortex” during descent. It has now 
been shown that the adult stem of Platyzoma is at points protostelic (62) ; from this 
state medullation arises by purely intrastelar changes. In like manner, an inner 
endodermis is developed around the pith. In default of any evidence of loss of stelar 
gaps and of cortical origin for the pith and inner endodermis, a reasonable interpreta- 
tion of the adult stelar structure may be that the medulla and inner endodermis are 
intrastelar in origin as in location. As in Schizaea malaccana and Schizaea dichotoma 
so also in Platyzoma, the possession of inner endodermis locally or throughout the 
adult medullated protostele increases the endodermal area, and may enhance the 
physiological control. 
The ontogeny of Gleichenia pectinata * has been studied, and the full facts of 
progression from the protostele of the sporeling to the solenostele of the adult are now 
known. At the base of the juvenile plant is a long protostelic tract bearing a number 
of leaf-traces (a, b, c, d, e, text-fig. 3). A transverse section of the stele at the point 
of departure of the fourth leaf-trace is shown in fig. 25. There is a solid parenchy- 
matous xylem-core (X), around which the phloem forms an unbroken ring (Ph.). 
There is no endodermal pocket in the succeeding sections, and the leaf-trace departs 
in the protostelic manner. In some of the smaller plants the upper portion of the' 
protostele showed a mixed pith (fig. 26), with large tracheides (T) scattered through 
thin-walled parenchyma. In some instances the stele was in this condition im- 
mediately below the growing point ; in others a definite pith existed. In all such 
cases examined the ontogenetic progression to the mixed pith or pure medulla was 
marked by segregation of tracheides and parenchyma, as in Schizaea malaccana, and 
the stellar endodermis was maintained unbroken. Thus the pith is an intraxylic 
tissue. The protostelic tract of the juvenile axis is followed in the ontogeny by a 
long medullated stage (text-fig. 3, levels I to II), the central region of tracheides 
being entirely replaced by parenchyma. At first there are no xylic gaps at the 
departure of the leaf-traces or elsewhere. Fig. 27 shows the transverse section of the 
stele during the protostelic departure of a leaf- trace (f) and a root- trace (g) in this 
region. Later the xylem is interrupted above the leaf-traces, and opposite the xylic 
gaps thus formed there may be slight pocketing of the endodermis (h, text-fig. 3). 
The ontogenetic stelar changes already traced for Schizaea malaccana are thus in 
general repeated. As the stele is followed forward, phloem appears in the pith as an in- 
definite and incomplete ring of sieve-tubes lining the xylem internally (III, text-fig. 3). 
These are shown at Ph.' (fig. 28), which represents a transverse section of the 
stele during departure of leaf-trace (k). A solenostelic structure soon follows, 
* The materials for this study were kindly provided by Mr Vm, Harris, F.L.S., Director, Hope Gardens, 
Jamaica. 
