i6o 
FRANCIS E. LLOYD 
through circular perforations, recalhng the similar behavior in the tannin 
idioplasts of the persimmon (Lloyd, 191 1). In the course of time the 
parenchyma cells completely collapsed, and the mucilage had then reached 
its maximum hydration permissible under the circumstances. The prepar- 
ation was now stained in alcoholic safranin and the few mucilage cells re- 
maining unbroken were stained. On being placed in water nearly all of 
these subsequently burst under microscopic observation. 
Inclusions in Mucilage Cells 
Starch is generally found within the protoplast sensu stricto, in amounts 
usually correlated with the amount found in the neighboring cells, but some- 
times in less quantity.*^ It is perhaps unexpected to find that in a much 
shrunken frond of Opuntia susquehannensis, already referred to, the old 
and fully hydrated mucilage cells contained a very large amount of starch, 
as did the remaining parenchyma. No evidence of an inverse quantitative 
relation between the amount of starch present and the extent of mucilage 
secretion could be observed. This starch, it would seem wholly probable, 
was laid down after the mucilage had been secreted. This view suggests 
the question of the physiological condition of the mucilage cells after the 
amount of mucilage is sufficient to cause displacement of the protoplast 
from its usual and conceptually normal position, namely, against the cel- 
lulose wall. Specifically, does the protoplast become moribund and even- 
tually die when compressed within the swollen mucilaginous mass? With- 
out attempting at the moment to answer this question, it may be pointed 
out as bearing on it, that the size of the mucilage cell does not appear to 
remain fixed after the amount of mucilage has become sufficient to press 
the protoplast into a relatively small compass within its interior. It is 
certain, at any rate, that there is no disappearance of mucilage cells during 
the developmental phase of the frond, and it is similarly certain that the 
size of these cells in the mature tissue is much greater than in young, quasi 
embryonic material. Thus, in a frond some centimeters long, the mature 
mucilage cells near the growing tip measured 0.15 mm. in diameter. Toward 
the middle of the frond they measured fully 0.5 mm. in diameter, a gradual 
increase in size being observable as the eye receded from the growing apex. 
In view of the possible secretion of starch above mentioned, it seems pos- 
sible that in spite of the crowding of the protoplast by the hydrated mucilage, 
it remains alive, and that the cell grows. In this event, the pressure on the 
cell wall which causes stretching is dominantly the imbibition pressure of 
the mucilage. 
General Discussion 
A review of the literature pertaining to the matter under present treat- 
ment shows clearly that the essential features of the topography of the 
^ It is also found in the mucilage ("gum") cells of tragacanth, as noted by earlier 
observers. 
