258 Holmes.— Observations on the Anatomy of 
are easily distinguished by their size and shape from the wood fibres, and by 
their walls, which are thicker, pitted, and more completely lignified than 
those of the latter. The small vessels near the cambium are very thick- 
walled, with a quite circular lumen. Spirally thickened vessels occur in the 
protoxylem next to the pith. In longitudinal section the vessels are seen 
to consist of short segments communicating by round holes, chiefly in radial 
oblique walls ; the segments of the wide vessels may be four or five times 
Fig. 4. 
as long as they are wide, but are often shorter. The wood is richest in 
vessels immediately round the pith, especially in the four groups of primary 
xylem corresponding with the opposite and decussate phyllotaxy. In all 
except the smallest sections, where the widest vessels occur in these 
primary groups, there are wider vessels in the region of the wood just out- 
side this part, and they get smaller again towards the periphery, and more 
scattered. Apart from the primary bundles, the distribution of the vessels 
is very uniform in most of the sections, especially at the lower levels. 
