59 ^ hilL — The Histology of the 
PL XXXII), and very soon after this the calius becomes 
more diffuse, stains faintly and finally is dissolved away. 
The median nodules also lose their refringence in these 
old sieve-plates and are not so conspicuous when stained with 
iodine solution, and disorganization gradually takes place 
which on the solution of the callus becomes complete. Thus 
finally only the original network of the unaltered pit-closing 
membrane, which now stains deeply with methylene-blue, 
is left, and no trace of the original middle-lamella membrane, 
stretching across the meshes of the network or of the nodules, 
could be found (Fig. 13, PL XXXI). The old and disused 
sieve-plate appears therefore to be a distinct sieve with open 
pores, which is contrary to Strasburger’s figure 1 where he 
shows the middle lamella of the pit-closing membrane stretch* 
mg between the pieces of the unaltered radial wall, and so 
shutting off the sieve-tubes from each other. 
The cell-walls of the older disused sieve-tubes are much 
thinner than those of the active zone, apparently through 
loss of water, and, like their pit-closing membranes, they stain 
quite deeply with methylene blue owing to the infiltration of 
pectic substances. 
The Sieve-Plates in Surface- View. 
The appearance of sieve-plates in radial sections has been 
described by De Bary 2 , Strasburger 3 , Russow (Figs. 1 and 2, 
PL XXXI) and others, and figured by the two former in- 
vestigators. It seems, from what I have been able to observe^ 
that these descriptions and figures refer to optical sections of 
sieve-plates and not to surface-views between which there is 
some difference. 
It has already been pointed out how difficult it is to see 
traces of the actual slime-strings in the callus-rods in trans- 
verse section, when Russow’s or Strasburger’s methods are 
employed, owing to the similar colouration their reagents give 
with protoplasm and callus; yet in the pictures of surface- 
1 Strasburger, ‘ Bot. Pract. / 1897, Fig; 103 c. 
3 De Bary, ‘ Comp. Anat. ,’ Fig. 77. 
5 Strasburger, ‘Bot. Pract./ 1897, Fig. 101. 
