HilL — The Histology of the Sieve-Tubes of Angio sperms. 275 
tinuous across the membrane, thus suggesting a further analogy with the 
sieve-fields (Fig. 34, PI. XVIII). After a short time, however, the boring 
out appears to be complete, and a string of equal breadth enclosed by its 
sheath of callus unites the contents of the adjoining tubes (cf. Figs. 36 and 47, 
PI. XVIII). It has been frequently noticed that the pit areas at the ex- 
tremities of the large inclined sieve-plates of Vitis are occupied by threads 
of the definite sieve-field type, having median nodes and small callus-rods 
exactly like those on the lateral walls (Fig. 46, PI. XVIII). Such observa- 
tions are not without interest in connexion with the analogy which has 
been shown to exist between these two forms of connexions. 
Further, the sieve-fields may often undergo a subsequent boring after 
they have reached their normal stage of development. The pores are 
widened apparently by renewed ferment activity and the median nodal struc- 
ture vanishes (Figs. 45 and 48, PI. XVIII), so that to all intents and purposes 
a small sieve-plate has been developed as a secondary formation from a 
sieve-field. All these cases lend colour to the view that the sieve-fields 
and sieve-plates undergo similar developmental stages, but that in the case 
of the latter structures the changes have been more far-reaching. 
The gradual accumulation of callus on both sides of the sieve-plates in 
Vitis takes place in the manner already described for Wistaria or Cucurbita , 
and the paths of the slime-strings are left as fine tubes after the disorganiza- 
tion of the strings themselves has taken place, i.e., when the slime disappears 
from the tubes on the approach of the resting period (Fig. 54, PI. XVIII). 
Good figures of the sieve-plates in the winter condition are given by 
Wilhelm, 1 and in two cases (PI. VII, Figs. 89 and 90) he shows lines cross- 
ing the callus-masses. As the material was collected o'n April 27, these 
lines may represent the new slime-strings in the old pores of the callus- 
masses. 2 
Re-opening of the Sieve-Plates in Vitis . 
Vitis differs from the plants already mentioned in that its sieve-tubes 
are functional for more than one year. The sieve-plates, therefore, which 
become blocked by the callus in the ordinary manner in the autumn have 
to be reopened in the course of the coming spring. In order to obtain evi- 
dence of the time and mode of reopening of the plates, two plants of Vitis 
of different varieties were examined by means of sections taken at short 
intervals of time from February until the end of May in 1901, and some 
similar observations were made in 1904. The plants were grown in the 
open at the Cambridge Botanic Garden. It was noticed that at the begin- 
ning of April the buds began to swell, and on cutting into the stem the vine 
bled. The sieve-tubes and callus-cushions, however, appeared to be in the 
1 Wilhelm, 1 . c., p. 37. 
3 Perrot, Le Tissu crible, 1899, Figs. 20, 21, and 32, copied from Lecomte. 
