290 Scott and Brebner . — On Internal Phloem 
the spaces -(filled with parenchyma) between the bundles, and 
must therefore have a definite relation to the bundle-system. 
In this plant, and in many others, the outgoing bundle leaves, 
as it were, a parenchymatous space above its exit, which is 
only closed over further up by the converging bundles on 
either side. This space is the primary ray. In Acantholimon 
there is no interfascicular cambium, the ray following the 
growth in thickness by extension of its cells, perhaps 
accompanied by a few irregular divisions. No secondary 
rays are formed at any stage which has come under our 
observation. 
A normal fascicular cambium is formed between xylem 
and phloem and for some time the thickening goes on in a 
perfectly typical way. The annular and spiral vessels of the 
primary wood are succeeded by a secondary xylem, which is 
especially rich in vessels. These are of two kinds : large 
vessels with bordered pits, and much smaller ones with a con- 
spicuous spiral thickening and small pits between the coils. 
The closely packed elements with lignified thickened walls, 
which in a transverse section might easily be taken for 
woody parenchyma, are in reality these small vessels. The 
parenchyma, which is scattered among them, is entirely thin- 
walled and usually unlignified. In addition to these elements, 
the secondary wood contains scattered strands of fibres 
thickened so that the cavity has all but disappeared. The 
larger vessels occur in rather regular concentric layers. The 
phloem is very thick. It consists of elongated cells equivalent 
to phloem-parenchyma, but prosenchymatous in shape, of 
sieve-tubes, with inclined terminal sieve-plates, and their 
companion-cells. The first anomalous formations generally 
appear in stems about 2 mm. in diameter, where there is a 
wide zone of secondary wood and the outer cortex has been 
already cast off by periderm. The first departure from normal 
structure consists in the appearance of an internal cambium. 
This has not hitherto been observed. The cambial divisions 
may occur immediately within the protoxylem of the bundles 
in the outermost small-celled tissues of the pith, but in other 
