594 Hill.— -The Histology of the 
given by the darkly stained, irregular granules, and the dark 
dots seen in them are, as we have already seen, the real nodes 
of the threads. 
An examination of a callus-cushion of an older sieve-tube 
in surface-view shows a large number of stained points 
scattered irregularly over the surface (cf. Fig. 14, PL XXXII), 
which are the ends of the slime-strings radiating out through 
the callus ; an optical section shows appearances similar to, 
but less distinct than those already described. 
The Cause of Sieve-Plate Development. 
The structure of the sieve-plate has been described at some 
length in the preceding pages, and it now remains to be seen 
whether any explanation can be offered of the complicated 
developmental changes through which the sieve* plate passes 
before it attains to its mature condition. For we have 
seen that as soon as the alteration of the threads commences, 
the callus also appears in the form of little basin-shaped 
masses on the free surfaces of the small secondary pits, 
enveloping the altered portion of the £ connecting threads ’ 
(Fig. 7, PL XXXII ; Figs. 17-20, PL XXXIII), and the 
further changes in both proceed simultaneously ; moreover, 
the appearance of the median nodule is coincident with the 
completion of these changes ; so that it seems only reasonable 
to suppose that these simultaneous changes are all connected 
together and must be due to the same or similar causes ; and 
there seems to me no doubt but that they must be attributed 
to ferment-action. 
The presence of ferments in sieve-tubes has long been 
known owing to their peculiar effect on the starch-grains in 
the tubes, which stain a pinkish colour in consequence when 
treated with iodine solution. Another excellent example of 
ferment-action in sieve-tubes is afforded by the large holes 
occupied by thick slime-strings in the sieve-plates of Angio- 
sperms, such as Cucurbita, which have been bored out through 
the pit-closing membrane apparently along the lines of ‘ con- 
necting threads 5 ; and in addition to this evidence the final solu- 
