108 Oliver. — On the Obliteration of 
stains a rich golden-brown in corallin-soda, and in both cases 
in which I have found it well-developed callus-plates have 
been present. One of the specimens showing it [taken in 1779] 
had a small wound which cut through the zone of sieve-tubes 
on one side only. No doubt this cut was inflicted during the 
life of the plant, for the sieve-tubes cut into all showed this 
curious condition for some millimetres on either side of the 
cut. The undamaged tubes were quite normal. This con- 
dition is no doubt a pathological or protective arrangement. 
Any further account of the contents and changes of these 
sieve-tubes must be reserved until I have an opportunity of 
examining fresh or properly preserved material. 
Callus commences to develop early in the history of the 
sieve-tubes. At the time of perforation, so far as I can 
ascertain, the sieve-plates are not callous, hence differing from 
the cases of Vitis, &c., in which the plate is slightly callous 
even before perforation \ Soon enough the sieve-plate be- 
comes invested in a complete covering of callus, not only on 
the upper and lower surfaces of the cellulose-framework, but 
also lining the pores. The sieve-plates would appear to be 
in this condition during the greater part of their functional 
activity, becoming obliterated from a further later development 
of the callus. If a fairly old stem be examined, in which the 
zone of sieve-tubes is about eight sieve-tubes broad, all the 
older and inner ones will be found to be completely callous ; 
this will apply probably to the inner five rings. The sixth 
and seventh will as a rule be slightly callous, but not yet 
obliterated, and the outmost tubes of all will show no callus- 
formation of any kind. This arrangement is a fairly constant 
one. In very young stems only the inmost will show any trace 
of callus. The trumpet-hyphae, however, become obliterated 
much earlier, i. e. by the time that two or three rings of true 
sieve-tubes are formed. In Fig. 19 an early stage in the de- 
velopment of the callus (from M. Orbignyana ) is shown. The 
callus (stained red) is formed all round the cellulose-frame- 
1 Cf. Wilhelm, loc. cit. 
