Bower.—Studies in the Phytogeny of the Filicales. VII. 29 
of a central pith, a ring of tracheides, usually continuous, but irregular in 
thickness and in the size of the tracheides. 1 Outside this is a zone of con¬ 
junctive parenchyma with phloem, which is at first very scanty: and 
externally an ill-defined pericycle, and continuous, well-marked endodermis, 
with cells containing mucilage. When a root-trace departs there is no inter¬ 
ruption of the tissues. What happens is, in the case of the medullated 
stele, that from the outer margin of the xylem-ring—which has previously 
thickened in preparation—a supply of tracheides comes off, and moves 
peripherally, and the outer tissues yield. When fully separated from the 
xylem of the stele, the phloem, pericycle, and endodermis curve inwards 
behind the outgoing root-supply. Finally the endodermis from either side 
meets, and the root-supply is thus shut off from the stele without any 
opening to the external cortex. 
Similarly with the young leaf-trace. Here, however, where the stele is 
already medullated the xylem-ring opens, and a sector of it with one free 
margin swings outwards; thus continuity is established between the central 
tissue and the external conjunctive. The endodermis and pericycle yield 
as before without any break of continuity, encroaching upon the external 
cortex. Presently the outward-moving sector is detached from the xylem 
of the stele and moves obliquely outwards, while the endodermis and peri¬ 
cycle close in behind it, completing the abstriction. Thus in the case both 
of root and leaf the stele is throughout completely closed. This condition 
is maintained until a number of leaves have been produced, and the stele has 
attained considerable size. Meanwhile the quantity of phloem has increased, 
and it is distributed both inside and outside of the xylem-ring ( Lindsaya - 
condition). The two are in continuous connexion whenever a leaf-trace 
departs. 
The transition from this Lindsay a-coY\d.\X\on to solenostely is the next 
step. It may be described with illustrations in the case of a definite plant 
cut as Series E. The departure of the last leaf-trace given off on this 
relatively simple plan is shown in Fig. 23, I. The endodermis still 
forms a complete barrier surrounding the obliquely outward-swinging leaf- 
trace. Certain elements in the central region of the stele are developed 
as sieve-tubes, which are now present in considerable numbers, in addition 
to those of the external phloem. The structure of the whole stele 
of the same plant after this leaf-trace has been abstricted is shown at 
a higher level in Fig. 23, II, from which it is seen that the stem retains 
the Lindsay a-stmctmo. to that point. The next higher leaf-trace which 
comes off is shown in Fig. 23, III. Here the origin of the leaf-trace and 
its relation to the stelar tissues are the same as before, but the obliquity 
is more marked, and the leaf-trace itself larger. With this goes a deeper 
involution of the endodermis, followed by the thick-walled cells of the 
1 Compare Sablon, 1 . c., Fig. 4. 
