122 
INTERCELLULAR RELATIONS OF PROTOPLASTS. 
with small but gradually enlarging callus projections, exactly 
harmonising in position on the two sides, which projections 
indicate the position of the future perforations. According 
to Russow, these callus formations are first made in the 
pre-existing depressions in the wall, while Janczewski* and 
Wilhelm believe that these local thinnesses show only after 
the projections have developed to a certain extent. All three 
observers agree, however, that at this period the original 
cellulose membrane is no longer simple and plain, but is now 
composed of a network of thicker parts, with a mesliwork of 
thinner, the thinner parts lying between the corresponding 
projections in question, the network representing the parts 
free from these projections. As to the source of the callus, 
there is another divergence of opinion. Wilhelm says, 
somewhat hesitatingly, that it arises from a local transforma¬ 
tion of the cellulose ; Janczewski affirms this with positiveness; 
Russow, on the other hand, claims that it is in no way the 
result of cellulose transformation, but is in all cases a new 
deposit from the protoplasm of the sieve tubes. Quantitatively 
it appears improbable that the thick callus which many 
sieve tubes possess should be entirely a transformation of the 
original extremely thin cell wall; equally improbable does it 
seem to be that the protoplasm should form new cellulose 
deposits only to have them at once reconverted into callus. 
Such cellulose deposits would have to be either internal or 
external, but of these there is never the slightest sign.f The 
deposit of callus can go on slowly for considerable periods ; 
Russow thinks that the amount deposited is dependent on the 
length of period during which the sieve perforations remain 
open, and the sieve tube thus active. But, on the other 
hand, it is quite clear that in Monocotyledons, where the 
sieve tubes remain active for the greatest length of time, there 
is a smaller formation of callus than in Dicotyledons, where 
the sieve tube is seldom active for more than two or three years. 
* Janczewski in his earlier observations held the same opinion as 
Russow, viz., that the wall was pitted before the formation of the 
callus. 
t As callus is but a modified form of cellulose it is more probable 
that both processes go on, the original callus formation, so far as it 
involves the thinning out and local disappearance of the original cell 
wall, beiug due to transformation, subsequent increase being due to 
callus deposit. Transformation, however, if it takes place, is appa¬ 
rently not simple. Many of the reactions of callus are distinctly 
semi-protoplasmic. Some suggest a resemblance, probably however 
merely casual, to the basis substance of the cell-nucleus—nuclein (a 
name first given to it by Hanstein, although Miescher and Hertwig had 
both previously employed the term, and equivalent to the chromatin of 
Flemming, and Kernsubstanz of Strasburger). 
