252 Hill ’ — Histology of the Sieve-Tubes of Angiospcr ms. 
effected by stained callus-threads ; a surface view may sometimes be seen 
showing separate blue dots which are his callus-threads, and Figs. 31 and 
32, PL XIV, illustrate this stage in the development. 1 The network or 
framework of the sieve-plate continues to project still more above the 
general surface of the membrane owing to the progressive conversion of its 
substance into callus, and at the same time the perforation of the several 
sieve-fields takes place. The mucous contents of the sieve-tubes begin to 
bore through the centre of the membrane of each field from one side only; 
this boring is not due to the fusion of the callus-threads which traverse the 
membranes of the sieve-fields, for it commences with a small hole in the 
centre of the field, and this gradually increases in diameter until the whole 
of the field area is occupied by a slime-string. By means of these 
strings the slime masses of adjoining sieve-tubes are in communication 
(cf. Figs. 33-35, PL XIV). The conversion of the thickened portion, or 
network, of the sieve-plate into callus still continues, and there results 
eventually a narrowing of the several pores (Figs. 36-37, Pl. XIV). 
It may be noted from the figures (Fig. 36, PL XIV ; Figs. 38 and 39, 
PL XV) that during the progress of this narrowing the callus has increased 
in size, whilst the cellulose or substance of the original membrane has 
decreased. Finally the thickening callus masses of the framework fuse 
together and the shutting of the sieve-plate by a callus-rod is complete 
(Fig. 40, PL XV). In a longitudinal section of the callus-mass the remains 
of the original membrane can still be seen as a series of nodes, and in 
a surface view, on the solution of the callus, this same primary membrane 
is left behind as an open network (Fig. 42). 2 
The finer network seen in Pinas , representing the remains of the sieve- 
field membrane (cf. Figs. 26 and 27), is therefore absent in Wistaria , and the 
network of the latter and of Angiosperms generally is considered to be 
homologous with that larger network which encloses the sieve-fields in Pinas. 
Strasburger regards the callus in Pinas as derived from the swellings of 
the ends of the changed protoplasmic threads, whilst in the sieve-plates of 
Wistaria and Angiosperms he considers that callus is formed by the trans- 
formation of the substances of the main network of the cell membrane. He 
thus agrees, in the main, with Lecomte as to the mode of origin of the 
callus-pads in Angiosperms, and alters the opinion previously advanced 
in the ‘ Leitungsbahnen ’, that the callus formation in Angiosperms and 
Gymnosperms must follow the same line of development. That a substance, 
so constant in its occurrence as the callus, and found in such diverse groups 
1 It seems necessary to point out here that, although his Figs. 29-32 are all drawn to the same 
magnification, the surface views Figs. 30 and 32 do not appear to correspond with their respective 
sections, Figs. 29 and 31. It is also difficult to understand Fig. 30, and in Fig. 31 the callus-rods, 
into which the protoplasmic threads are supposed to change, are not indicated. 
2 Fig. 42, PI. XV, shows such an empty plate in surface view : but it is not easy to understand 
its correspondence with the section in Fig. 41. 
