Hill . — The Histology of the Sieve- T ubes of A ngio sperms. 273 
callus is dissolved away by ferments, and only the cellulose mesh or reticulum 
of the plate is left (Text-fig. 9). 1 
Cucurbit a. 
The course of sieve-plate development described for Wistaria appears 
to obtain in Cucurbita also. The sieve-areas are here very large, and when 
young and only lightly covered by callus show little holes as though they 
had been pierced by pins. In some cases the pit-processes of adjoining 
sieve-tubes were united by definite threads or fine strings which appeared to 
pass through the holes (cf. Text-fig. 6). 
It is important to point out that the size of the individual sieve-areas or 
pits varies considerably in any sieve-plate ; those near the centre being 
larger as a rule than those near the periphery. It naturally follows that the 
smaller the area the fewer will be the threads which it contains, and in fact 
in the case of some of the smallest of these areas there seems to be only a 
single thread. Such cases as these afford a link between the ordinary sieve- 
fields and the larger sieve-plates, and, moreover, in some plants it appears 
probable that each area of a sieve-plate is provided with but one thread. 
The further history of the sieve-plate agrees closely with Wistaria. Large 
holes are formed in the meshes of the plate and there is a thin callus lining 
to each hole. After a period of full activity callus is gradually laid down 
over the sieve-plate to form a conspicuous pad, and the slime-strings, though 
they still remain thick, are considerably lengthened (Fig. 19, PI. XVII). 
With the approach of autumn the callus masses are more pronounced, and 
the slime-strings become attenuated towards the periphery of the callus- 
cushions, so that each string is somewhat fusiform in shape and a surface view 
shows a small string in the middle of each callused sieve-area (Figs. 20 and 22, 
PI. XVII). Finally, the slime-strings become broken up and cease to func- 
tion as connexions, though the holes through which they passed still exist as 
such in the old callus-pads, and with appropriate methods their paths may 
be seen either as clear lines or more commonly marked by lines of dark 
granules, representing the remains of the slime-strings (Fig. 21, PI. XVII). 
The presence of pores in the old callus-masses appears to have escaped 
general notice, 2 for, according to the usually accepted view, the callus covers 
over and obliterates all trace of the slime-strings on the approach of winter. 3 
There is, however, no doubt that the holes or tubes do persist in the 
oldest plates, and their gradual attenuation seems to form quite a sufficient 
obstacle to the translocation currents. The importance to the plant of the 
1 Cf. Strasburger in Jahrb. fiir wiss. Bot., xxxvi, 1901, PI. XV, Fig?. 41 and 42. It is unfortunate 
that in these, as in some other figures, the proper correspondence between the surface views and 
sections is not clearly shown. 
2 Cf. , however, Lecomte, 1 . c., p. 266, and Figs. 5, 6, and 13. 
3 Wilhelm, Beit. z. Kennt. d. Siebrohren, Leipzig, 1880, PI. VII, Figs. 80, 81, 85 &c. ; 
Strasburger, 1 . c., PL XV, Fig. 40. 
