384 Darhishire . — Observations 071 Mamillaria elongata. 
have seen them penetrate as far as 3 mm., but at this point they are much 
reduced in quantity. The libriform cells of the root agree in every way 
with those of the stem in structure and form. 
We find that the tracheids of the root with their characteristic annular 
thickening are gradually replaced by the spiral tracheids of the stem. In 
some cases intermediate stages are found, giving the appearance of 
reticulate tracheids. Cork is extensively formed in this region. It lines, as 
already mentioned, the passages through the cortex made by the adventitious 
roots, and also covers the roots for a very long distance. 
Root-hairs may occur in large quantities on the root, often obscuring 
very largely the growing-points of the young lateral roots. They very 
soon apparently lose their absorptive function and then may resemble 
fungal hyphae. But I was unable to detect the presence of any mycorhiza 
at all. 
In the older roots the proportion of wood to parenchyma in the 
metaxylem is different to what it is in the younger root. There is in the 
former more lignified tissue, and it is therefore harder and tougher. 
In Melocactus , v. Breda de Haan notes that the protoxylem of the root 
consists of spiral vessels ( 4 , pp. 9, 10) and the metaxylem of scalariform 
vessels ( 4 , p. 12). 
(c) Anatomy of the Tubercle. 
The arrangement of the tissues in the tubercles or warts which form 
such a prominent feature of the Mamillariae is extremely characteristic and 
interesting. Each tubercle is roughly of the shape of a stunted cone, the 
blunt apex of the latter being crowned by a marginal ring of spines with 
one central one (PI. XXV, Figs. 1, 2, 3). 
We can distinguish first an epidermis, which covers the whole tubercle 
(PI. XXV, Fig. 15, e). The cells are flattened and have wavy outlines. The 
cuticle is not very thick, namely about i*5ju. The epidermis contains no 
chloroplastids. It is interrupted by fairly numerous stomata (PI. XXVI, 
Figs. 9, 15). These are of the typical cactaceous type ( 28 , p. 459). 
Several subsidiary cells are found running parallel to the guard-cells. 
In a transverse median section the guard-cells are seen to be at the 
same level as the other epidermal cells and are not depressed. A 
small ledge of cuticular wall projects in such a way as to produce a small 
antechamber which leads to the actual passage between the guard-cells. 
This leads to the internal air-chamber with which the whole very extensive 
system of intercellular air-spaces inside the tubercle communicates. 
Disregarding the spines and the cushion on which they are inserted, 
the remaining tissues of the tubercle are ground-tissue and vascular tissue 
(PI. XXVI, Fig. 15). 
