268 Hill . — The Histology of the Sieve-Tubes of A ngio sperms . 
fields is easier to study than in Vitis, since the threads are fewer, larger, and 
well separated from each other. In a surface view of the wall of a young 
sieve-tube, the individual strings, each enclosed in its callus-sheath, can be 
clearly seen (Fig. J 5, PL XVII), whilst in a section the paired callus-rods with 
their contained slime-string, together with the node at the middle lamella, 
are no less conspicuous after treatment with Safranin and Water Blue 
(Fig. 14, PL XVII). 
Precisely similar courses of development appear to obtain for the 
lateral sieves of Tilia , Cucurbita , and other Angiosperms. In Cucurbita 
the development of the callus-rods and the appearance of the median node 
are very clear, for the young sieve-field, viewed in section, is seen to be 
studded with little knobs of callus at the surface of each fine thread 
(Fig. 25, PL XVII). 
Lateral sieve-fields may be found both in the radial and the tangential 
walls, and are usually distributed in a similar manner in either wall, but 
in Tilia , and probably in some other plants, the threads are far more 
abundant in the tangential than in the radial walls (Fig. 59, PL XVIII), 
in some cases being so numerous that a surface view shows the tangential 
wall covered with dots (Fig. 61, PL XVIII). In general structure these 
threads appear to be similar to those of the smaller groups in the radial 
walls, but owing to their close crowding it is not easy to determine the 
exact relations of the callus to the individual threads. 
Viscum differs from what appears to be the normal arrangement in 
some interesting details ; the threads in the walls between two sieve- 
tubes are seen in surface view to be grouped in narrow fusiform areas, 
with which the walls are closely packed, and the wall appears to be full 
of threads. In section the threads are seen to be in small pits in groups 
of three to five, and neither in summer nor in winter can any callus be 
demonstrated in these lateral walls. The threads do not stain very 
deeply, and there is no appearance of a node at the middle lamella 1 
(Figs. 26 and 27, PL XVII). 
The Median Node. 
If these observations are correct, it seems probable that the conspicuous 
node at the lamella is in some way a function of the slime-string and callus- 
rod, since in Viscum , where the lateral threads do not become converted 
into slime-strings, the node is apparently absent. 
A slight enlargement at the middle lamella is often noticeable on the 
threads of ordinary cortical cells, e. g. in the cortex of Cucurbita , or on 
the threads of the thick cell-walls of Endosperms, 2 but in such cases the 
staining of the thread throughout is of the same character, and the minute 
1 Kuhla figures a node on all these threads. Bot. Zeit., lviii, 1900, PI. Ill, Figs. 12-14 and 17. 
2 Gardiner and Hill, Histology of Endosperms ; Proc. Camb. Phil. Soc., xi, 1902, p. 449, 
PI. V, Figs. 13, 15, &c. 
