264 Hill. — The Histology of the Sieve-Tubes of A ngio sperms. 
are enclosed like small islands in the general parenchymatous tissue 
(Fig. 27, PL XVII). Viscum also differs markedly from the other plants 
which have been examined, both in the form and position of the companion 
cells, and also in the distribution and character of the connecting-threads 
between them and sieve-tubes, and in the lateral walls between adjoining 
sieve-tubes. 
The Companion Cells. 
The anatomical relation of the companion cell to the sieve-tube is too 
well known to need any remark. 
VYlillCl Willi 
callus-rods crossing the wall, and small callus- 
cushions. (Copied from Wilhelm, PL IV, 
Fig- 350 
1 Wilhelm, 1. c., Pis. I and II, Figs. 13, 
2 Cf. Perrot, 1. c., p. 62. 
In the case of Vitis, excellent figures are 
given by Wilhelm 1 (cf. Text-figs. 1 
and 4), and there is also a short resume 
of the subject by Perrot. 2 Two of 
Wilhelm’s figures of longitudinal sec- 
tions are of especial interest (viz., Figs. 
26 and 35), for the numerous pits in 
the wall between the companion cell 
and its sieve-tube are very faithfully 
drawn, and the addition of connecting- 
threads in the pits is only needed to 
make his picture complete. In longi- 
tudinal section the pits in these walls 
appear to be very narrow, but when 
seen in surface view they are found 
to be extended transversely almost 
across the whole face of the com- 
panion cell to form narrow fusiform 
areas, over which the short threads are 
distributed either uniformly or aggre- 
gated more or less into small secondary 
groups. The histology of the connecting- 
threads will be referred to later. 3 
The Bast Parenchyma Cells. 
Between the bast parenchyma cells 
and both the sieve-tubes and the com- 
panion cells there are small pits of 
connecting- threads similar in character 
to those which connect the cells of the 
bast parenchyma themselves. The pits 
are more or less circular in outline 
16, 18-20; PI. Ill, Fig. 26; PI. IV, Fig. 35. 
3 Cf. p. 279. 
