I22 BACTERIA IN RELATION TO PLANT DISEASES. 
few final touches on a good leather strop. ‘The maintenance of good edges on 
microtome knives is a matter of great importance and considerable difficulty, and 
where much material is to be cut it is very economical of time to send away such 
knives to be put in order by some 
expert. In recent years the writer 
has sent all such knives to Charles 
Lentz & Sons, Philadelphia, with 
very satisfactory results. Knives 
suitable for serial sections are shown 
in fig. 114 AandC. In fig. 114 B is 
shown one of a set of knives not in- 
clined to spring and well adapted to 
the cutting of hard material with a 
long slant stroke. ‘These knives were 
made to order by Lentz & Sons ata 
cost of about $6 each. An end-on 
view of all these knives is shown in 
fig. 114 a, b,¢,d.. 
Many plant tissues, especially ma- 
ture leaves, are full of very hard cal- 
cium oxalate crystals, and the difficul- 
ties of properly cutting such material 
are very great. The cutting of thin 
sections of bone would be quite as 
easy. After even a few sections the 
edge of the knife looks like a minia- 
ture saw and the sections themselves 
are badly torn, partly by the dulled 
knife and partly by the movement of 
Fig. 112.* the crystals themselves. In case of 
the yellow disease of the hyacinth the 
writer has never been able to make satisfactory thin sections, many of the soft cells 
being filled with bundles of very hard raphides which he has not been able to 
dissolve without serious injury to 
- tissues. In such cases thick 2/4B Vb Biccsbe t 
ree-hand sections are about all D000 of Plum 
that can be hoped for. x ® 00 IGXAE 
Serial sections are cut on the DOOD] | Fine cavity 
microtome. ‘The one shown in xX xX X0 in parenchym 
pl. 13 and fig. 119 leaves nothing 
to be desired in the way of a 
perfect-working durable instrument, The ribbon-carrier is above the table at the left. 
The knife is stationary. The block moves up and down, and the razor-carrier 
Fig. 113. 
*Fic, 112—A page from the paraffin record-book. The numbers on the slide-boxes (fig. 103) 
correspond to numbers in this book. ‘Two-thirds actual size. 
+ Fic. 113—A mounted slide of serial sections, showing manner of labeling. 
44 
