274 Hill . — The Hisiology of the Sieve-Tubes of A ngio sperms. 
persistence of these pores in the callus-pads will be the more evident 
when the question of the re-opening of the sieves after the winter’s 
rest is considered. 
Vires. 
The large inclined sieve-plates of Vitis which next claim attention are 
really compound structures, since they are made up of several small plates 
(Fig. 47, PL XVIII), each of which is constituted like the horizontal plates 
of the sieve-tubes of other plants. Two methods of sieve-plate development 
appear to obtain in Vitis ; in the one, the course of events seems to be closely 
similar to that already described, and distinct indications of little groups of 
four to five threads have been seen in each sieve-area (Fig. 30, PI. XVIII). 
In one instance there was fairly clear evidence of threads in the walls 
between the paired callus-basins (Fig. 31, PL XVIII). In other cases, 
however, there can be little doubt that there is never more than one thread 
to each sieve-area, so that each small sieve-plate is quite comparable in its 
general structure to a sieve-field. Paired knobs of callus, owing to ferment 
e. 
Fig. 10. Fig. ii. Fig. 12. 
Diagrammatic sketches to show the development of the slime-strings from single threads : — 
Fig. 10. A young pitted sieve-plate (cl.), with single threads (//,) in each pit. 
Fig. ii. The commencement of ferment action, showing the enlargement of the ends of the 
threads to form slime-strings ( s . s.) and the formation of the callus basins (<:.). 
Fig. 12. The final shape, in which the thread has been bored out and enlarged, and the callus 
change has proceeded right across the membrane and forms a coating (c.) to the cellulose 
sieve-plate (cl.). 
action, are now formed on either side of the pitted membrane at the ends 
of each thread, and the change gradually spreads across the membrane 
until a slime-string in its callus-rod or sheath is formed (Figs. 32 and 33, 
PL XVIII, and Text-figs. 10 and 11). Several cases of a dark dot between the 
two opposed callus-knobs have been noticed (Fig. 32, PL XVIII), which may 
represent the node of the original thread in course of alteration, 1 the pieces 
of the slime-string itself having been retracted from the callus owing to 
imperfect preservation of the material. In surface view such sieves show 
a small dot in the centre of a small callus-area (cf. Fig. 33, PL XVIII, wdth 
the sieve-fields of Wistaria, Fig. 15, PL XVII). The slime-strings so 
formed soon begin to enlarge, but a swollen node is often seen at the centre 
of each string, and in such cases the callus does not appear to be con- 
1 Cf. with the node on the threads of the sieve-fields (Figs. 14, 16, 18, PI. XVII, and Figs. 45 (b), 
59, PI. XVIII). 
