160 STEWART: MUCILAGE FORMATION IN THE CACTI 
periphery of the protoplast in the plasma membrane, but that no 
crystal was present. The cytoplasm is pressed toward the center 
of the cell as the mucilage increases until it remains as a mere 
remnant in the interior. By the use of a strong sugar solution he 
was able to make the mucilage surrounded by its delicate plasma 
envelope pull away from the wall. He observed striations in the 
mucilage, but made no attempt to account for their origin. 
Walliczek (23) claims that the mucilage of the cacti arises as a 
secondary thickening of the primary cell wall. He also states that 
it does not give a cellulose reaction in the moment of its formation 
nor later. He suggests that the stratification in most cases is 
dependent upon a different water content in the layers. This 
stratification, he says, shows best when the material is preserved 
in alcohol and water added later. 
Longo (11, 12) holds that the peculiar structure of the mucilage 
which Walliczek describes is due to the action of the alcohol which, 
he believes, withdraws water from the mucilage. He used fresh 
material as well as alcoholic preparations and found that the muci- 
lage was not the result of a transformation of the cellulose mem- 
brane, but came from the protoplasm, showing the characteristics 
of mucilage as soon as it appeared. He does not agree with Lau- 
terbach (8) that the mucilage is produced in droplets in the parie- 
tal protoplasm. He finds it appearing between this and the thin 
cell, wall of cellulose, which never undergoes any modification. 
As to whether the mucilage arises from the wall or from the 
protoplasm, the opinions seem to be about equally divided, but 
Walliczek, Lauterbach, and Longo all agree that it is accumulated 
between the plasma membrane and the cell wall. In this con- 
nection I might say that in the present paper I have figured a case 
in which the wall between a mucilage cell and the neighboring 
cell shows the middle lamella with the wall of equal thickness on 
each side of the lamella, which is good evidence that the wall is 
not affected by the formation of the mucilage. 
For the most part I have studied Rhipsalis rhombea (from the 
New York Botanical Garden), the species in which Schleiden (20) 
said he could not find any cells filled with mucilage. Owing to the 
incompleteness of his record it is impossible to determine whether 
