382 Dar bishire. — Observations on Mamillaria elongata . 
here, but my examination has been very cursory with regard to this 
question. 
I will here merely describe the structure of a younger, and then that of 
an older root. 
We can take first a young lateral root with a diameter of about 1 mm. 
The roots which I examined were all pentarch, with the exception of one, 
which was tetrarch (PI. XXVI, Figs. 33, 34). 
A few parenchymatous cells in the centre of the root, about eight to 
twelve in number, form the pith, in the outer part of which lie the small groups 
of protoxylem, consisting of 5-8 elements (PI. XXVI, Figs. 31 ,32,/). The 
cells of the pith are more or less round in transverse section, but rather 
elongate in longitudinal view. They measure about 12-18 by 1 00-150 /x. 
Their ends fit on to one another square, i.e. at right angles to the longi- 
tudinal direction of the cell, and the cells themselves contain a fair amount 
of cytoplasm and a distinct and fairly large nucleus (PI. XXVI, Fig. 32). 
The protoxylem-elements appear to be tracheids and not vessels. 
They are annular throughout. They measure about 10-15 M across, hut I 
have been unable to ascertain their length. The thickened ring is 1-5 to 
2 fj. thick, and projects about 1-5 /x into the cell-cavity. The distance from 
one ring to the next is very regularly about 8 The very thin unthickened 
part of the tracheidal wall consists of cellulose. 
Radiating outwards from the protoxylem-bundles may be seen the 
primary medullary rays. The cells of the primary medullary rays are 
usually much compressed tangentially, measuring occasionally as much as 
50 /x in radial direction, and 19 jx in tangential direction. They become less 
compressed towards the periphery of the transverse section. The medullary 
ray as a whole naturally becomes slightly broader towards the outside, the 
rows of living cells radiating outwards very regularly. Finally they pass 
into the cortical tissue (PI. XXVI, Fig. 31). 
The large wedge-shaped masses of metaxylem fill up the space between 
the medullary rays. The elements of the metaxylem are tracheids and 
wood-parenchyma. Both are arranged in regular rows, but the distribution 
of the two kinds of wood-elements varies. Some of the radiating rows will 
consist almost entirely either of tracheids or parenchymatous cells, or both 
may be equally represented. The parenchymatous cells, whether merely 
xylem-parenchyma or belonging to the secondary medullary rays, are 
smaller than the tracheids. Their diameter varies between 10 and 20 /x ; 
their length, like that of the cells of the primary medullary rays, may reach 
as much as 200 /x. They are of course living cells. The tracheids exhibit 
annular thickening throughout, although in a few cases two neighbouring 
rings may be connected by a band of thickening — thus forming a short 
spiral. They measure 10-26 /x in diameter, and seem to attain a length of 
200 y, but only in a few cases could the transverse walls be made out. The 
