Vegetal dissemination in the genus Opuntia. 
J. W. TOUMEY. 
The discussion here presented is based upon observations 
made during the past four years on the various species of 
Opuntia indigenous to Arizona. All of these plants, more 
especially the younger growth, are soft and fleshy, with large 
quantities of sap stored in the cortex and pith. No other 
plants so persistently retain their moisture when once secured. 
A thick epidermis with small sunken stomata and the evapor- 
ating surface brought down to a minimum, by the condensed 
form characteristic of the entire genus, enable them to remain 
green for months, even when continually exposed to the dry 
and scorching heat of our southwestern plains. They are 
alike at home on the plains and among the rocks of our 
mountains and foot-hills. Although of recent origin, com- 
pared with other large families of plants, they have lived for 
ages subjected to an environment which has placed them among 
the greatest economizers of water to be found in the entire 
vegetable kingdom. Not only are they economizers of wa- 
ter, but their tissues are, under ordinary circumstances, well 
provided with water, even after months of exposure to an aver- 
age maximum temperature of 100° F. and untouched by. dews 
or rains. 
Much of the tissue entering into their composition, con- 
sists of large thin-walled parenchyma cells, which serve as 
store-houses of water. These cells have the power of taking 
nent. As the days become warmer and the rains less fre- 
quent these cells gradually give up their contained moisture 
and the joints become withered and wrinkled. This process 
is, however, very slow, as Many species remain green a year 
or even longer, after €very source of outside moisture has 
been withdrawn. 
The first of July, 1892, half of one of the flat joints of 
Opuntia basilaris was brought to my room and placed, with- 
out soil or moisture, in a small open box. The specimen was 
[356] 
