Hill . — The Histology of the Sieve-Tttbes of Angiosperms. 265 
and fairly deep, and the pit-closing membrane is pierced by a small group 
of some five to seven threads, as seen in section, which usually stain darkly. 
Between a sieve-tube and an adjoining bast parenchyma cell only a few 
such pits will be seen, though in a single sieve-tube of Vitis as many as ten 
little pits have been counted. In winter material small hemispherical pads 
of callus are found covering these pits on the sieve-tube side, and with 
appropriate treatment the threads or their remains can be seen fanning out 
to the margins of these little callus cushions (v. Fig. 49, PI. XVIII). 
Between the bast parenchyma cells and companion cells small pits of 
threads are fairly numerous, and appear to be similar in every respect to 
those between two parenchymatous cells, 1 and no callus is formed over 
them in winter material. As was noticed in the case of P inns l callus is 
formed normally only in the sieve-tubes themselves. 
The pits in the walls of the phloem medullary ray cells are very 
numerous, and are occupied by small groups of threads similar to those of 
the phloem parenchyma and of parenchymatous tissue generally. 
Bast Fibres. 
Bast Fibres are a further constituent of the phloem commonly to be 
found, and may occur in definite bands alternating with the bands of 
sieve-tubes, as in Vitis (cf. Text-fig. 4) and in Tilia , or with a somewhat 
irregular distribution as in Wistaria. In Viscum and Cucnrbita bast fibres 
are apparently absent. 3 
When young their protoplasmic contents are in communication by 
means of connecting-threads, both amongst themselves and also with 
adjoining parenchymatous tissue. In Vitis the gradual development of 
the thick cell-walls has been followed ; the final result is the formation 
of deep funnel-shaped pits with thin pit-closing membranes, which are 
traversed by a number of short and delicate threads (Fig. 53, PI. XVIII). As 
long as the walls of these sclerous cells are increasing in thickness they 
retain their protoplasmic contents and their connecting-threads, but on the 
completion of the secondary changes they become dead and empty, and 
the perforations of the membranes either remain as empty holes or, more 
probably, are obliterated. 
THE DEVELOPMENT OF THE SIEVE-FIELDS AND 
OF THE SIEVE-PLATES. 
The Sieve-Fields. 
The term ‘ Sieve Fields ’, in the sense in which it is used in this paper, 4 
denotes the groups of fine connecting-threads or strings which are found 
normally on the lateral walls, and which serve as a means of communication 
1 Wilhelm, 1 . c., PL IV, Fig. 36. 
3 v. Perrot, pp. 76 et seq., and Figs. 59 and 60. 
T 
2 Hill, Ann. Bot., vol. xv, p. 597, &c. 
4 v. p. 251 , footnote 6. 
