528 Rivet t. — The Anatomy of Rhododendron ponticum , L., and of 
narrows upwards, and a wide pith which increases relatively towards the 
apex. It is hard and firm at the base, but becomes soft and flexible in the 
leafy region. 
The wood, in transverse section, is seen to consist of dead lignified 
elements, interspersed with living cells, and divided radially into narrow 
sectors by the medullary rays. The lignified elements include small thick- 
walled fibres and wide water-conducting elements. The first-formed wood 
(protoxylem) abutting on the pith consists of very small water-conducting 
elements, with thick walls, sometimes in process of disintegration. The 
water-conducting elements of the metaxylem are larger and occur in 
irregular groups just behind the protoxylem. The paucity of fibres in the 
primary xylem is very evident. At the period of growth at which it is 
functional, the supporting material of the stem is provided by a peripheral 
ring of hard fibres in the pericycle which are developed very early. Thus 
the first-formed wood is almost entirely a response on the part of the plant 
to the necessities of water-conduction. As the stem grows older, the proto- 
xylem becomes squashed and obliterated, and elements of the metaxylem 
also lose their function, forming brownish patches. The secondary xylem 
occupies the main part of the wood cylinder. Peripherally the conducting 
elements are small and separated by wide patches of fibres, as is character- 
istic of autumn-formed wood, but for the inner two-thirds of the wood the 
numerous wide lumina of the water-conducting elements are very con- 
spicuous. The lumina are very variable in shape, though many are elongated 
radially. The walls, though lignified, are comparatively thin and delicate, 
compared with those of the fibres and living cells. In transverse section it 
can be seen that the water-conducting elements communicate with living 
cells by bordered pits, with considerable thickening of the middle lamella. 
There are also shallower, less obviously bordered pits between adjacent 
conducting elements. The scattered living cells are more or less square in 
section, but where the living cells form uniseriate medullary rays they are 
wider radially than tangentially. Most of the rays are uniseriate, except in 
the neighbourhood of a leaf-insertion, where wider patches of living cells and 
numerous transitional tracheidal elements are to be found. All the living 
cells in the wood have rather thick walls and stain lightly with safranin. 
They have living protoplasm and a nucleus, but contain little starch at this 
season of the year. 
In longitudinal section the water-conducting elements of the proto- 
xylem are found to be elongated vessels with spiral thickenings. They 
reach a length of J cm., as can be more easily seen from macerated material. 
The segments forming the vessel are joined end to end without obvious 
remains of cross-walls, the perforations being large and entire. These 
elements of the protoxylem contrast strongly with the water-conducting 
elements of the secondary xylem, which are made up of segments about 
