Sieve-Tubes of Finns , 589 
already noticed, are apparently composed of protoplasm like 
those of the younger sieves, judging from their staining 
properties with iodine, safranin, and other dyes, and from their 
general appearance of granularity, &c. : they are as before 
quite fine and faint, and each thread possesses a small median 
dot (Fig. 8, PI. XXXII (b)). 
In the youngest thick-walled sieve-tubes 3 , however — i.e. 
the boundary-cells (Fig. 9, PI. XXXII (a )) — ‘the connecting 
threads 5 may be similar to those of the younger elements just 
described, or, what is more likely, may show the commence- 
ment of certain secondary changes, which alter their appear- 
ance and staining properties in a striking manner (Fig. 9, 
PI. XXXII (b)). In pit-closing membranes about this region 
the individual threads do not exhibit the uniformity of 
structure throughout their length which usually characterizes 
‘ connecting threads,’ for part of the thread is now much more 
noticeable, being thicker and staining more darkly than the 
unaltered portion of the protoplasmic thread. This change, 
which commences at the free surfaces of the threads, usually 
affects the halves of the threads on one side of the middle 
lamella first, and then in the same way attacks the halves on 
the other side ; so that the young sieve-plates at this stage 
often present a peculiar appearance, with the portions of the 
threads in one-half of the pit-closing membrane fairly thick 
and darkly stained, whilst in the other half they are only 
faintly stained and much less conspicuous. 
All stages in the alteration of the character of the sieve- 
plate threads have been noticed. The change is seen to 
commence on the surface of one side of the pit-closing 
membrane, and gradually to proceed inwards along each 
thread to the middle lamella ; then the threads on the other 
side of the membrane appear to be similarly affected ; and the 
change which can be easily observed in the stained sections 
1 Cf. pp. 579 and 587. 
2 A large number of sieve-plates at this border zone have been examined 
in sections of material, which has been fixed and stained in various ways, and in all 
cases similar results have been obtained, and the unsymmetrically stained threads 
have been clearly seen. 
