STEWART: MUCILAGE FORMATION IN THE CACTI 163 
the cytoplasm in a rather characteristic fashion (Fic. 4). In 
some cells the mass of mucilage is rather homogeneous throughout, . 
in many others it is reticulated and zoned (Fic. 7), so as to have 
led some investigators to believe that it is the much thickened 
and stratified cell wall. 
While the cytoplasm with its included starch disappears 
entirely the cell wall nowhere shows a breaking down or disorgani- 
zation. At the end of the process it is just as thick as at the 
beginning. In favorable cases the middle lamella is discernible 
and in such preparations the secondary wall is seen everywhere 
intact and is just as thick and no thicker on the side of the lamella 
next the mucilage cell than on the opposite side (Fic. 6). This 
would seem to show that the wall does not become disorganized 
or changed in any way as the formation of mucilage proceeds. 
The evidence all seems to point in one direction, namely, that 
mucilage in the cacti is formed not at the expense of the cell wall, 
but at the expense of the cytoplasm and nucleus. Neighboring 
cells, as well as the mucilage cell, perhaps, contribute of their 
content. They at any rate become flattened and contain very 
little cytoplasm (Fic. 5). 
Haberlandt (5) has pointed out that in many cases slime con- 
tainers may serve as water reservoirs and that it is not necessary 
to regard such slimy cell content as a useless excretion. He notes 
that the early differentiation of the slime cells in the vicinity of 
the growing point appears to point to their having a mechanical 
function in the growth process, and suggests that we have to do 
here with ‘‘Schwellapparaten.” 
This observation is perhaps borne out by experiments by 
MacDougal and others at the Tucson laboratory. It will be 
recalled that in his work on colloid hydratation and growth in the 
cacti, Long (10) reported that, in general, growth and swelling 
paralleled each other rather closely. Later MacDougal (13) found 
that agar, composed of pentoses presumably having some qualities 
identical with those of the plant mucilages, and both young and 
old disks of Opuntia will swell more when placed in distilled water 
or an alkaline solution than when placed in an acid solution. He 
found also that the apical parts of joints showed greater capacity 
for absorption than the basal portions. Ina series of similar imbi- 
