250 
Davie . — The Structure and 
it springs. Soon after the adaxial strands have passed into the base of the 
petiole their inner edges become extended, so that ultimately each of these 
strands is curved in form, with a dilated abaxial portion and a delicate strip 
of xylem projecting sometimes almost parallel to the lower surface of this, and 
forming a hook, with its back towards the periphery of the petiole (Fig. 6, a). 
Two or three protoxylem groups are found in the positions /r#. 1 , prx?, &c. 
The whole of the hooked mass of xylem is surrounded by phloem, which, 
however, thins out or altogether disappears on the side of the mass facing 
the centre of the petiole. A several-layered pericycle and a single brown- 
coloured layer of endodermis enclose each strand, while often a mass 
of sclerenchyma, one or two layers deep, encloses the whole bundle. 
The median main bundle has a narrow plate of xylem with its long 
axis parallel to the adaxial side of the petiole, and with one or two proto- 
xylem masses on the side facing the centre of the petiole. Phloem is 
present generally only on the side facing the periphery of the petiole. 
Sometimes a very small amount of phloem is developed also on the side 
facing the centre of the petiole. 
The smaller subsidiary strands have the same structure. 
Small groups of sclerenchymatous cells are here and there seen in the 
tissue of the petiole, generally between the subsidiary strands, but often also 
between them and the main strands. Other groups of sclerenchyma are 
found scattered in the ground tissue of the petiole. 
When the supply to the pinnae is about to be given off, one of the 
adaxial strands of the petiole extends towards the adaxial corner of the 
petiole, lengthening the narrow strip of xylem on its adaxial side and push- 
ing a tongue of xylem towards the pinna (Fig. 6 , b). Some distance from 
the back of the hook two swellings occur in the xylem, one in the narrow 
portion, the other in the wide portion. These soon form a bridge, cutting 
off an island of parenchyma, the ring of xylem surrounding which has the 
shape of a signet ring in section, until at last the strand of the pinna-trace 
goes off as a ring of xylem with a larger amount of tracheides on one side 
than on the other (Fig. 6, c). Then the wider side of the ring pushes 
up a tongue of xylem, a gap appears, filled with parenchyma, the two 
halves of the strand draw apart to right and left, leaving a channel of 
parenchyma between them (Fig. 7, III). These take on the shape of 
the adaxial strands of the petiole. From one of them is nipped off a 
median strand of the shape of the early pinna-trace. And from it and 
from the other prosrachial 1 strand come two subsidiary strands. The final 
1 As the faces of the rachis of the pinna are not orientated with regard to the stock but with 
regard to the rachis of the frond, the terms ‘ adaxial ’ and ‘ abaxial 1 cannot be used in connexion 
with the pinna-trace strands. It is therefore necessary to use new terms whereby these strands may 
be distinguished from one another. ‘ Prosrachial ’ means directed towards the rachis ; ‘ aporachial 
directed away from the rachis. 
