582 Hume . —The Histology of the Sieve Tubes of Pteridium 
Conclusions. 
According to de Janckzewski, as summarized by Russow , 1 the sieve 
tubes of Vascular Cryptogams differ from those of Phanerogams in their 
shape, in the absence of callus, and in the fact that the pores of the 
sieve plate are always closed. Russow was able to show that Cryptogams 
are not characterized by the absence of callus from their sieve tubes, and 
the foregoing account shows that the pores are not closed, but are as open as 
they are in Phanerogams ; and that the process of development is similar 
in many ways. The outstanding differences therefore are in shape and 
contents ; the sieve tubes of Vascular Cryptogams are larger and thicker 
walled and contain refringent granules. 
The size of the sieve tubes and the thickness of their walls in Vascular 
Cryptogams are, it would seem, very likely dependent on the fact that, owing 
to the absence of secondary thickening, they have to function for a long time, 
in some cases for as much as twenty years, while in some Dicotyledons and 
Gymnosperms they are renewed each year. In some Phanerogams (as in 
Vitis), it is true that they do function for more than one year, after being 
blocked with callus in the winter. Vitis , being a climber, perhaps needs more 
than the average amount of phloem, and therefore makes use of more than 
the current year’s growth of phloem. It was suggested to the writer that the 
absence of blocking by callus in winter in Pteridium might be due to the fact 
that the rhizome is an underground organ, and is therefore not so severely 
affected by the frost as an aerial one would be. With a view to determining 
whether this theory is borne out by the state of affairs in other underground 
organs, a rapid examination was, made of winter material of the roots and 
stem of Vitis vinifera. The sieve tubes of the stem showed the great pads 
of callus blocking the sieve plates which Hill has described , 2 but in the 
roots no such pads could be found, and the sieve tubes showed masses 
of slime pressed against the sieve plate, the pores of which were clearly 
traversed by the slime strings. Little doubt, therefore, seems to exist 
that the sieve tubes of Pteridium remain unblocked by callus in winter 
because the rhizome is a completely subterranean organ and is, on that 
account, not subjected to the severities of weather to which an aerial organ 
would be exposed during the winter. 
The facts ascertained concerning the finer histological details of the 
sieve tubes of Pteridium aquilinum correspond closely with Hill’s observations 
on Pinus and the Angiosperms. The presence of the protoplasmic threads in 
the early stages, and the way in which they are bored out to form slime- 
strings associated with callus, are all materially the same. The salient 
differences are the ultimate total disappearance of the callus while the sieve 
1 Russow (’82), p. 208. 
2 Hill (’08), pp. 275-6, and PL XVIII, Figs. 41 and 54. 
