ORIGIN AND NATURE OF THE MUCILAGE IN THE CACTI 
The number of mucilage cells in the cortex may be smaller per unit 
volume of tissue than in the medulla, but on the other hand there is fre- 
quently realized a condition in which both cortex and medulla are crowded 
with them. In a joint of Opuntia susquehannensis about 3.5 cm. long, and 
which, though small, was shrunken as if it had been long deprived of water, 
the mucilage cells were so large and numerous as to occupy much more 
than half the total volume of the whole. They were moreover crowded 
upon each other to such an extent as to approximate a lacunar condition 
such as Walliczek (1893) described for Tilia. Trecul (1875) believed this v 
to be true also for cacti, etc., but in view of the peculiar difficulties of obser- 
vation a reasonable question of fact may be permitted. 
Place and Time of Origin 
Though occurring within both medulla and cortex, the mucilage cells 
arise first in the medulla and later in the cortex. The earliest may be found 
in the medulla directly beneath the growing point, while in the cortex the 
youngest readily recognizable as passing into the more obvious condition of 
a definitive mucilage cell could be found only as far as 4 mm. from the apex, 
in the Opuntia above mentioned. Relatively few, however, originate 
within the actively expanding region at the apex of a young joint. They 
arise rather during the whole period of growth in all regions of the enlarging 
joint, young ones being found even toward the base of a joint several centi- 
meters long. They are therefore secondary in their origin, and before they 
assume their special character are indistinguishable from the surrounding 
cells, whether of cortex or medulla. It is a legitimate speculation that the 
numbers of mucilage cells may be modifiable under various environmental 
conditions.^ In Carnegiea gigantea the mucilage cells are not to be found 
in the palisade tissues and are considerably fewer in number than in Opuntia. 
They are absent from the younger tissues, none being found by me in a 
small individual 45 cm. tall, except below the level of ca. 30 cm. from the 
apex. 
Mode of Origin, Chemical Structure and Inclusions 
The mucilage cells as such are at first recognizable only by their size. 
When once differentiable, one notes that the nucleus and nucleolus enlarge 
to an enormous size. At first parietal in position, the nucleus usually 
becomes central. The protoplasm also increases in amount both absolutely 
and relatively, and the nucleus becomes suspended in many thick strands. 
Chloroplasts and starch grains are usually present, and a large stellate 
cluster of calcium oxalate crystals is frequently, though not invariably, to 
be seen. 
The wall is at first indistinguishable from the walls of surrounding cells, 
but when considerable size has been gained it becomes somewhat thickened. 
^According to Tschirch {fide Walliczek, 1893, p. 274), the mucilage content of the 
marshmallow is greater when the plant is grown in dry soil. 
