STEWART: MUCILAGE FORMATION IN THE CACTI 161 
or not his Rhipsalis rhombea was identical with the species which 
I used. My plants are all from greenhouse sources and were 
probably introduced from Europe. Besides Rhipsalis rhombea, 
I have used Rhipsalis pachyptera and Rhipsalis Houlletiana, 
brought from France, likewise Opuntia inermis and Pereskia 
Pereskia. 
In the leaves of the flower buds of Opuntia and of Rhipsalis, 
the mucilage cells are often so large and so numerous that in cross 
section these leaves seem to be almost filled with mucilage. 
With the Flemming triple stain the mucilage is colored blue, but 
is not so deep a blue as the starch grains. The mucilage is never, 
with the method I have used, of the same color as the wall sur- 
rounding it. 
In the flower buds of Rhipsalis the large mucilage cells are 
abundant, not only in the floral leaves but also in the ovary wall, 
in some cases almost every cell being filled with mucilage. The 
mucilage cells are more numerous in the periphery than toward the 
center. Sections were made also of very young Rhipsalis stems. 
In these meristematic tissues there are numerous mucilage cells 
which are two or three times as large as the neighboring cells. 
The adjacent cells (Fic. 1) are typical vegetative cells containing 
a large central vacuole surrounded by a thin layer of cytoplasm 
adhering to the cell wall. In it are suspended the nucleus and a 
few starch grains. 
On the other hand, in the large cell (Fic. 2), before any mucilage 
appears, the cytoplasm is much more dense. While it is spongy 
and vacuolated, there’is no large central vacuole, neither is there 
any starch as arule. The cells at this stage contradict entirely 
the possibility suggested by Lloyd (9) that their large size is due 
to imbibition of water by the mucilage which they contain. In the 
species studied by me, the cells destined to form mucilage reach a 
diameter two or three times that of adjoining cells before the 
mucilage begins to form in them. In Fic. 3 the protoplast is 
surrounded by a layer of mucilage, but its diameter inside the 
mucilage layer is approximately the same as that of the protoplast 
in Fig. 2. 
The growth in these cells at this stage is true growth and not 
at all a mere matter of increased water content of a central vacuole. 
