Sieve-Tubes of Finn's . 
593 
views, given by them, some four or five darkly stained and 
fairly large points are shown in each sieve-field, which I have 
been unable to see when examining real surface-views by 
their methods. If, however, an optical section is taken, the 
dark points can be seen and appear to be nothing else but 
the enlarged median nodes of the slime-strings. By our 
methods this view receives confirmation, for in a real surface- 
view a certain number of fine, regular, faintly stained points 
can be seen arranged usually round the edge of each sieve-field 
(Fig. 15 A, PI. XXXII) ; but on focussing, so as to obtain an 
optical section, the appearance of the sieve-plate changes, and 
the little fields, which usually appear more numerous near 
the lamella, are now seen to be occupied by fewer, larger, and 
more darkly stained dots than those of the surface-view. Thus, 
whereas at the surface some 4-9 slime-strings may be seen in 
each field, at the lamella only from 3-5 nodes occur in the 
smaller and more numerous areas (Fig. 15 a', PI. XXXII). 
The difference between the number of the sieve-fields in 
a surface-view and an optical section of the same sieve-plate 
explains the description given by Russow of the developing 
callus-rods, which often, he says, stretch towards the middle 
lamella as ‘double brown lines’ 1 (Fig. 9, PI. XXXI). For 
a callus-rod, which appears as a single sieve-field at the 
surface of the sieve-plate, frequently divides, and at the 
lamella two sieve-fields, or callus-rods, as seen in section, are 
found. Thus if there are ten sieve-fields at the surface of 
a sieve-plate, it is quite likely that as many as fifteen may be 
seen when an optical section is taken. 
This fact of the division of the callus-rods also explains 
why the slime-strings of the mature sieve-plate are frequently 
found running in obliquely placed groups in the pit-closing 
membrane (cf. Fig. 11, PI. XXXII), since the callus-formation 
proceeds along the groups of connecting threads. 
An appearance like that of Fig. 16, PI. XXXII is often 
seen in an optical section of a mature sieve-plate stained with 
safranin alone: in this case the appearance of sieve-fields is 
1 Russow, loc. cit., 1882, p. 281, and cf. p. 579. 
