57 ^ Htirne.—- The Histology of the Sieve Tubes of P ter idium 
Histology. 
In the earliest stages of development of the sieve tubes of Pteridium 
aquilinum , the same difficulties of observation were experienced as seem to 
have been encountered by A. W. Hill The walls of the sieve tubes, at an 
early age, are thin (PI. LV, Fig. 29), and it is not until they have undergone 
quite a marked increase in thickness that it becomes possible to demonstrate 
the presence of threads traversing the walls. The contents of the sieve tubes 
are at first quite considerable, consisting of rather darkly staining proto- 
plasm, which tends to contract very easily with any reagents and, in trans- 
verse section, often falls out altogether. Such behaviour would probably 
rupture the connecting threads, but even in cases where the contents are 
still present and are little or not at all contracted, no certain indications can 
be seen of any connexions. As the diameter of the sieve tube increases the 
protoplasm becomes more and more drawn out into a thin layer, lining the 
walls, to which it is very closely applied. The walls are by this time con- 
siderably thickened, the areas of the sieve plates are mapped out, and in 
particularly favourable cases, delicate protoplasmic threads may be seen 
crossing the membrane (PL LIV, Fig. j). At the middle lamella they show 
a rather more darkly staining protoplasmic node. The recognition of the 
protoplasmic threads before the appearance of callus is extremely difficult 
in the large sieve tubes, but it is easier in the elements of the protophloem. 
These elements become very much thickened, and show deep pits with long 
drawn-out pit fillings. Callus often cannot be detected at all ; in fact it 
would appear as if the protophloem elements were not much used by the 
plant, and retain their juvenile characters right through life (PI. LIV, Figs. 2 
and 3). The connecting threads in the sieve plate are not as a rule 
aggregated into groups, but are dotted about singly all over the sieve plate. 
Up to, and even after, the first appearance of callus the refringent granules 
so often described appear to be small and distributed through the sieve tube, 
though mainly collected about the sieve plates (PL LIV, Fig. 1). It cannot be 
said with absolute certainty that, at this stage, they bear a definite relation 
to the threads, since they often appear to have no connexion with them ; 
how far this can be attributed to disturbance brought about by cutting, 
however, it is not very easy to say. 
The callus appears first as a shallow basin about the head of each 
thread (PL LIV, Fig. 4) ; callus formation seems generally to take place 
simultaneously on both sides of the pit-closing membrane, though some 
instances 1 were observed in which it was advanced further on one side than 
on the other (PL LIV, Fig. 4). Callus formation advances from either side 
towards the middle lamella, but stops short at it, while a corresponding 
1 Cf. Sykes (’08), PI. XIX, Fig. 15 ; and Hill (’08), PI. XVII, Fig. 24. 
