574 Hume . — The Histology of the Sieve Tubes of Pteridium 
gams was not very extensive, but he observed the presence of callus in 
most Ferns, with the exception of the Marattiaceae and Ophioglossaceae, 
and noticed that the refringent granules adhere to the free ends of the 
callus rods. He came to the conclusion that the sieve plates are perforated, 
but does not seem to have been able to demonstrate both callus rods and 
connexions at the same time. 
Poirault , 1 in the course of his extensive researches into the structure 
of Vascular Cryptogams, also came to the conclusion that the sieve plates 
must be perforated by connecting threads, but he too was unable to prove 
it; neither could he determine whether the paired callus plugs, on either 
side of the sieve plate, were or were not continuous with one another across 
the middle lamella. 
The question has remained in this uncertain condition ever since, as 
far as the Vascular Cryptogams are concerned. The only additional ob- 
servations worthy of note are due to Boodle , 2 who, using Grtibler’s azoblue 
as a callus reagent, showed the widespread presence of callus among the 
Ferns, with the exception of the Schizaeaceae, Hymenophyllaceae, and, as 
he at first thought, of the Gleicheniaceae. Examination of material of the 
rhizome of Gleichenia pectinata , which had not been allowed to come into 
contact with spirit, revealed the presence of callus , 3 and suggested the 
probability of its presence in the other two orders as well, if suitable material 
and methods could be found to demonstrate it. Boodle 4 mentions that the 
refringent granules are ‘ often distinctly grouped on the sieve plate ’. The 
question of the perforation of the sieve plates was not attacked by him, but 
the callus is described as a rod running through the wall, and having 
expanded heads. He also notes the presence of callus in the pits between 
sieve tubes and phloem parenchyma cells. 
In Pinns 5 and the Angiosperms , 6 A. W. Hill has traced the develop- 
ment of the connexions between adjoining sieve plates. He has shown 
that, in their earliest condition, these appear as delicate protoplasmic threads, 
with a darkly staining node. Later, probably by the action of ferments 
working along them, the threads become converted into thicker slime strings, 
while, at the same time, the cellulose of the cell-wall immediately surround- 
ing the slime string is altered into callus. Several slime strings may be 
contained in the same callus rod, as in Pinus , or each string may have its 
own callus rod ( Vitis). It is only the cellulose portion of the cell- wall 
which becomes converted into callus ; hence the callus does not form 
a continuous rod right across the pit-closing membrane, but is interrupted 
at the middle lamella. Here, however, the ferments also bring about some 
change, and immediately round each median protoplasmic node the pectic 
- 1 Poirault (’93), pp. 191-5. 
3 Boodle, loc. cit., p. 708. 
6 Hill (’01). 
2 Boodle (’01), pp. 395-9. 
4 Boodle, loc. cit., p. 397. 
3 Hill (’08). 
