Vessels of the Onion . 173 
cells being of course in an extremely young state and very 
short. 
4. Portion of leaf between the succulent base and the green 
tubular part . The very elongated cells show nodules of callus 
on the transverse septa. 
5. Piece of green tubular leaf just above the colorless 
sheathing part. Small callus-formations on almost all the 
transverse septa examined and occasionally on the side walls 
(Fig. 4). The next younger tubular leaf, about a quarter- 
inch below the apex, shows similar small callus-plugs on 
several of the transverse septa examined, though not on all. 
In the succulent leaf-scales of a resting onion there was 
callus on nearly all the transverse septa examined, and 
occasionally on the side walls ; but in the shoot enclosed in 
the centre, where the tissue is of course very young, there 
was none on the thin unpitted transverse septa. If, however, 
the onion is set to germinate, the leaves of the shoot which 
grows out soon show a formation of callus, which, is also found, 
just as before, in the cells of the succulent sheath-scales. 
From these observations it is evident that, except in the 
youngest stages, and even before the transverse septa become 
conspicuously pitted, callus-formation occurs upon them ; at 
first usually as small plugs, afterwards often of a more 
irregular form and sometimes spreading more or less over 
the whole plate. The complete covering of both sides, such 
as obtains in sieve-tubes during winter, is very rarely seen, 
one instance is noticed in Fig. 3, b. 
I have repeatedly tested sections from the different parts 
of leaves of all the ages and in all the conditions mentioned 
above, to see if the pits in the walls were ever perforated, 
using for the purpose (after Gardiner) concentrated sulphuric 
add in which methyl-violet was dissolved ; or chlorzinc- 
iodine followed by staining with picro-Hoffmann’s blue. The 
only cases in which perforation was observed occurred in the 
succulent leaf-scales of a spring-onion in which tangential 
sections from the lower half of the leaves showed the contents 
to be continuous through the swollen pits of the transverse 
