re ee Ce ee 













THE STEM | 57 
across, so that their thickened walls appear as a double-ring. 
In a longitudinal section of the stem, however, the vessels — 
are cut lengthways in half, so that the thickening of the cell- 
wall is seen in surface view. | 
~ 
pees 
seas en 
A 
i 
hy 
yy 
pol 
oe econ 

Fig. 25.—SunFLOWER, Fisro-vAscuLar Bunpie, LoNnGIruDINAL 
SECTION. (High power, somewhat diagrammatic. ) 
cr, cortex ; end, endodermis; /, fibres of pericyle and bast; s.t, sieve- 
tube; c, companion cells; », nucleus; cb, cambium ; p, pitted. vessel ; 
w.f, wood fibres ; sc, scalariform vessel] ; a, annular vessel ; s, spiral vessel. 
If the lignin were deposited in equal thickness over the whole 
cell-wall, then there would be no special markings; but the 
lignin is more thickly laid down in some places than in others. 
Sometimes the part of a cell-wall thus thickened is spiral in 
outline, then the vessel is called a spiral vessel (Fig. 25, s) ; 
and sometimes the thickening is laid down at intervals in 
rings, then the vessel is annular (a). Very often the thicken- 
ing is laid down over the whole cell-wall, leaving little gaps 
called pits, so that the cell-wall looks studded with depressions ; 
these are the thin places where the lignin has not been 
deposited. Such a vessel is said to be pitted (yp). The 
wood-fibres are prosenchymatous, and the wood-cells paren- 
chymatous, The wood of the Sunflower thus consists of: 
D 

