278 Hill . — The Histology of the Sieve- Tubes of A ngiosperms. 
As the question of callus formation has been discussed somewhat fully 
in an earlier paper 1 , there is no need to enter upon the subject again in any 
great detail. This research on the Angiosperms has tended to confirm the 
results already obtained, and there seems now little reason to doubt that the 
earliest formation of callus, which is coincident with the development of the 
slime-strings, is due to the alteration of pre-existing cellulose, whilst the 
superficial pads or cushions of the later-formed or secondary callus are 
gradually deposited by the protoplasm. 
Some account of the diverse views on callus formation has also been 
given in the introductory portion of this paper, and Russow’s interpretation 
of the shape of the callus-basins was mentioned. He considered that the 
polygonal outline of these basins or pits — as seen in a surface view of a 
young plate — afforded definite proof that the callus is laid down entirely by 
the protoplasm. There seems, however, very little reason for believing that 
the shape of these areas necessarily throws any light on their mode of for- 
mation. The pits of the young sieve-plate are at first more or less circular, 
then, as the whole membrane grows, they increase considerably in size, until 
finally the mesh or framework of the sieve-plate represents only a small pro- 
portion of the total area. As these changes proceed, the concomitant 
growth naturally causes the pitted areas to assume an angular outline in 
consequence of their mutual pressure (cf. Figs. 3, 4, 9, and 22, PI. XVII). 
It is at this stage, when the ground plan of the young plate has become 
fixed, that the callus makes its first appearance, and it appears to be due to 
the alteration of the surface layers of these polygonal pit membranes rather 
than to deposition by the protoplasm (cf. Figs. 1 and 2, PI. XVII, and 
Text-figs. 7 and 11). 
This view of the primary formation of the callus is also supported by 
the developmental history of the sieve-fields, where the callus appears as 
little dots or spherical heads round about the end of each connecting-thread, 
which, when closely crowded, often assume an angular outline. There 
seems, therefore, to be little reason to doubt that in all cases the first forma- 
tion of callus is due to the alteration of cellulose, by the action of ferments, 
in the immediate vicinity of the connecting-threads (Figs. 38-40, PI. XVIII). 
The Connexions between Sieve-Tubes and other Tissues 
in the Phloem. 
Reference has already been made to the companion cells 2 in their 
relation to the sieve-tubes, but nothing has been said about the character 
of the threads connecting these two elements. 
The simplest case is afforded by Viscum , where the connecting-threads 
in the walls between companion cells and sieve-tubes appear to be exactly 
similar to those in the lateral walls between two sieve-tubes. In both cases 
1 Hill, Ann. Bot., vol. xv, pp. 597-602, 605-606. 
2 Cf, p. 264. 
