Hill . — The Histology of the Sieve-Tubes of Angiosperms. 253 
of plants as the Brown Algae, Gymnosperms and Angiosperms, always in 
the same connexion with the perforations of the sieve-plates of the sieve- 
tubes, should have a different origin in the different groups, seems to be 
unlikely. My own results suggest that the mode of formation of the 
callus in connexion with the sieve-tubes takes place in the same way in the 
different groups of plants. 1 With regard to the smaller sieve-pits on the 
lateral walls of the sieve-tubes — the sieve-fields of Wilhelm 2 and others — 
Strasburger 3 considers that they differ but little from the pits in the walls 
of ordinary parenchymatous cells, and do not therefore agree in structure 
with the terminal sieve-plates. They differ from ordinary pits, however, since 
their threads are said to be transformed into callus, whose ends then swell — 
as he has described in the Conifers — to form callus-heads and eventually 
fuse together to form the lateral callus-pads (v. Fig. 44, PL XV). 
Thus it appears that in the same sieve-tube the callus-pads of the 
terminal and of the side walls have a different mode of formation. On 
the disappearance of the callus by solution there is no solution of the 
membrane separating the several callus-threads as was described in Pinns ; 
the membrane thus retains its original thickness and is perforated by empty 
canals (v. Fig. 45, PI. XV). In this respect, therefore, these lateral pits of 
Angiosperms differ from those of the Conifers in which, according to 
Strasburger, 4 a fine network is seen in the sieve-fields left naked by the 
solution of the callus ; for only the thickening layers have been dissolved 
and the middle lamella is left behind. The nodules therefore which he 
sees and figures in the middle of the sieves of young sieve-tubes are not 
swellings on the callus-threads, but represent the thicker parts of the middle 
lamella forming the fine meshwork. As this question has already been 
discussed in a former paper 5 it is unnecessary to go into details again here. 
Suffice it to say that the condition of the lateral thread groups of Angio- 
sperms, after the solution of the callus, appears to be essentially similar 
to the sieve-areas of Conifers. In both cases the membrane appears to 
be perforated by empty canals and the refractive nodules of the Conifers 
have vanished. Nothing 6 has been found to necessitate in any way an 
alteration of the views already put forward which tend to unite the 
1 Cf. the paper by Miss Sykes in this present number of the Annals. 
2 Wilhelm, 1 . c., Figs. 34 and 35, PI. IV. 
3 Strasburger, 1 . c., p. 531, Figs. 43-45, PI. XV. 
4 Ibid., p. 525, Figs. 26 and 27, PI. XIV. 
5 Cf. Hill, 1 . c., pp. 584, 592, Fig. 13, PI. XXXI. 
6 In the review of the paper on the sieve-tubes of Pinus, Strasburger, in Bot. Zeit,, vol. lx, p. 57, 
1902, adheres firmly to his views about the median nodules of the sieve-plates. Material has 
been examined which has been fixed in various ways, including alcohol material, and no trace can be 
seen of the conspicuous highly refractive nodes situated at the middle lamella between the paired 
callus-rods, after the callus has disappeared; cf. Hill, 1 . c., pp. 587-8, Figs. 11 and 13, PI. XXXI; 
and Fig. 21, PI. XXXIII. In the explanation to Fig. 11, PI. XXXI, p. 608, for nodules (mn) read 
(in), and for median nodes (in) read (mn). 
