354 COUTANT: WoOOD PERIDERM IN CERTAIN CACTI 
The pith is composed of thin-walled, isodiametric cells, loosely 
packed together, and with consequent large intercellular spaces. 
The cells themselves contain, aside from the usual protoplasmic 
contents, a considerable quantity of stored starch, and small 
crystals of calcium oxalate. These crystals are found in large 
quantities in practically all of the tissues of both opuntias, some- 
times causing, parallel with their own growth, an hypertrophy of 
the cells containing them. In a concentrated solution of hydro- 
chloric acid, the calcium oxalate of a medium-sized crystal does 
not dissolve for over half an hour. Lauterbach (2) describes these 
crystals as being star-shaped clusters composed of monoclinic 
prisms, which have a short principal axis, and states that they 
make up 85 per cent. of the weight of the ash. 
The bundles vary somewhat in their structure with the age 
of the plant and their position in the stem. Essentially, they are 
of the open collateral type, with hadrome and leptome on the same 
radius, separated by a cambial layer. The hadrome, much greater 
in extent than the leptome, consists of annular and spiral ducts, 
occasional parenchyma cells, and, in many cases, large interspersed 
masses of stereome tissue. The ducts and stereome cells are 
lignified, as shown by testing with phloroglucin and hydrochloric 
acid, and in cross section the latter greatly resemble the median 
optical view of the lignified-walled cells, to be described later in 
the formation of the periderm. The cells of the cambial region, 
as seen in cross section, present the typical brick-shaped appear- 
ance, and form, at the most, a layer not more than three or four 
cells deep. The leptome is composed of the characteristic sieve 
tubes and their accompanying companion cells, interspersed with 
parenchyma tissue. In cross section the peripheral portion of the 
leptome mass appears dome-shaped, and is capped by a mass of 
collenchymatic cells which resemble those of the hypoderm, re- 
ferred to later on. The walls of these cells often become more or 
less mucilaginous, an occurrence similar to that found by Lauter- 
bach (2, p. 262) in Pereskia, also one of the Cactaceae. The sub- 
cortical cells in the bundle region appear isodiametric, like those 
of the pith. However, there is a gradual merging of these cells 
with those in the outer cortical region, which are distinctly 
cylindrical and arranged end on end, with their long axes at right 
