595 
Sieve-Tubes of Pinus. 
tion of the callus-cushions is a further indication that ferments 
play an important part in the life-history of the sieve-tubes^ 
A ferment then we must assume, which is generated by 
the protoplasm of a developing sieve-tube of Pinus, enters 
the little pits and proceeds to attack and bore out the ‘ con- 
necting threads ’ of each group ; as it works along the threads 
towards the lamella its corroding action can easily be traced 
by the alteration of their staining properties, and the increase 
in their thickness which it causes (Fig. 9, PL XXXII ; Fig. 17, 
PI. XXXIII) ; at the same time or shortly afterwards the 
ferment may be generated in the adjoining sieve-tube of the 
same age, and enlarge the pores of the sieve-plate in the 
same manner until the lamella is reached, and so give rise to 
the complete slime-strings (Fig. n, PI. XXXII). But whilst 
the ferment enlarges the pores on the sieve-plate, it also 
affects the cellulose of the pit-closing membrane around each 
thread, and the threads being close together a callus-rod 
results, enclosing each group of slime-strings (Fig. 30, 
PL XXXIII; Fig. 11, PL XXXI). At the middle lamella, 
however, the membrane is not composed of pure cellulose but 
of pecto-cellulose, and the result of the ferment-action in this 
region is the formation of the highly refractive nodule 1 , which 
is thus seen to be strictly comparable in its mode of origin to 
the callus-rod formed from cellulose (Figs. 4, 10, and 11, PL 
XXXI). 
} In this connexion some of the effects produced on the median nodules with 
reagents are interesting ; they are stained darkly with iodine solution or Russow’s 
callus reagent (Fig. 6, PI. XXXII), and also by eosin (faintly), safranin, and 
benzyle blue, but with the two latter the staining appears to be of the nature of a 
surface colouration, and the colour is often so dark that the several nodules of 
the sieve-plate frequently look like a continuous, darkly stained median-plate 
(Fig. 11, PI. XXXII). With water-blue alone they are unstained though very 
prominent, but after the action of twenty-five per cent, chromic acid or ten per cent, 
potash they are stained purple in most cases. They also stain a bright blue with 
methylene blue after a few hours’ previous treatment with picro-nitric acid. After 
the action for some time of either ten per cent, potash, five per cent, nitric acid, 
twenty-five per cent, chromic acid, picro-nitric acid, or boiling water, they are 
still visible ; but after the action of one per cent, sulphuric acid made pale pink 
with a drop of weak permanganate of potash or of about eighty per cent, sulphuric 
acid (Russow’s second method) they cannot be distinguished. 
