380 Darbishire . — - Observations on Mamillciria elongata. 
close spiral, but does not project very far into the cavity of the tracheid. 
In a tracheid measuring 10/x across, the spiral thickening projected barely 
2 j u into the cavity, and even in a 20 ju, tracheid the spiral ledge was but 
a fraction thicker. Between these lignified tracheids we get a few thin- 
walled parenchymatous cells. These are of about the same size as the 
smaller spiral tracheids. 
The metaxylem is made up of a different form of spiral tracheid, but 
the same kind of parenchymatous cell met w T ith in the protoxylem. The 
spiral tracheids form a very characteristic constituent of cactaceous wood 
(PI. XXVI, Figs. 23, 25) and have already been carefully figured by Schleiden 
( 25 , PI. VII, Fig. 1, &c.). Subsequent authors have generally been satisfied 
with drawing them in a purely diagrammatic way or referring to Schleiden’s 
paper only ( 28 , p. 463, Fig. 91). 
These tracheids have angular walls where they adjoin other tracheids, 
but rounded convex walls where parenchymatous cells are their neighbours. 
50 /x probably represents the greatest diameter to which they may attain, 
the average lying between 30 and 40 /x for the larger ones, 20-30 /x for the 
smaller ones. In length they show a very uniform measurement, this 
being about 100 /x. The ends of adjoining tracheids meet at about the 
same level as was noticed already for the bast-cells (PL XXVI, Fig. 23). 
The ends are but slightly pointed or drawn out. Each tracheid has 
a cellulose wall, which is quite continuous, but from this projects a lignified 
spiral thickening of very large dimensions. The cellulose wall can plainly 
be made out and is probably about 1 /x thick. The spiral band of thickening 
projects into the cell-cavity a distance of 10-12 /x in the large tracheids, 
but never less than about 6 \x in the smaller ones. The spiral band is 
thinner at its point of attachment to the cell-wall and gets slightly thicker 
towards the centre. In longitudinal section we again usually see the 
cellulose wall bulging in towards the cavity of the tracheid when the latter 
is neighbour to a parenchymatous cell. The spiral thickening, however, 
always protrudes into the parenchymatous cell (PI. XXVI, Figs. 23, 25). 
The tracheids contain no protoplasm, as soon as they have become 
properly lignified. Frequently we can however see just inside the cambium 
tracheids which already show a large spiral thickening, which does not 
however respond to the ordinary wood-stains. Such cells contain cytoplasm 
and nucleus. 
Van Tieghem seems to maintain that the spiral tracheids of the 
Cactaceae invariably contain protoplasm and should therefore be called 
parenchymatous cells ( 28 , p. 462). It seems very likely, however, that his 
observations were made on some such younger tracheids as I have just 
referred to. These peculiar, broad and short, tracheids with their very well 
developed single spiral band form the bulk of the bundle of the main axis. 
The parenchymatous cells found in between the spiral tracheids are 
