754 Thiselton-Dyer . — Morphological Notes. 
walled cells, and b. is the intermediate thin-walled layer, 
besides which remnants of a second thin-walled layer are seen 
on the outside. On the lower side in the photograph the 
periderm is thinner, and includes only one zone of each of the 
two kinds of cells. In a. and b. the radial arrangement of the 
component cells is clearly seen, but in the thin-walled tissue 
a general crumpling of the cells has taken place to such an 
extent, that their original radial arrangement is quite obscured. 
The part of this layer marked b. shows most indication of 
radial seriation. Comparison of different sections, however, 
showed clearly that the whole of the periderm must have been 
formed by the same phellogen, for, wherever the periderm was 
least crushed, the same radial rows were seen to be continued 
through all its layers. Fig. 4 shows a small piece of periderm, 
from a section similar to Fig. 3, more highly magnified. The 
radial arrangement is clear, as the thin-walled cells are not 
much crumpled. These cells are nearly colourless, and 
suberized, while the thick-walled cells have yellow walls, 
which, perhaps with the exception of the middle lamella, are 
lignified through their entire thickness. The walls show con- 
spicuous stratification, and are provided with numerous pits ; 
the latter, however, are not represented in the drawing. 
The above characters agree well with the descriptions and 
drawings of cactaceous periderm given by Schleiden 1 and 
Arloing 2 . The tissue in question, when compared with the 
wound-periderm and normal periderm of a species of Cereus 
grown at Kew, was found to be practically identical with both 
in structure. 
The periderm is obviously a wound-periderm, which has 
been formed in the cortical tissues of the Cereus , so as to form 
a complete sheath enclosing the haustoria together with the 
adjacent injured cortical cells of the host-plant. The periderm 
was formed towards the haustorium,that is to say, the phellogen 
1 Schleiden, Beitr. z. Anat. d. Cacteen, Mem. de 1’Acad. Imp. des Sci. de 
St.-Petersbourg, 6° ser., tom. iv, 1839, P- an ^ Fig- 5 ( Echinocactus ). 
2 Arloing, Bouturage des Cactees, Ann. des Sci. Nat., Bot., 6 e s6r., tom. iv, 1876, 
p. 5, and PI. I, Fig. 2 ( Cereus ), & c. 
