MISSOURI BOTANICAL GARDEN BULLETIN 75 
tion, are uniformly near the surface, frequently not pene- 
trating the soil more than one or two inches, though extend- 
ing horizontally for long distances. While the length of 
time during which these strata contain sufficient moisture 
for absorption by roots varies greatly in different arid and 
semi-arid regions, it is probable that the period in general 
does not much exceed six weeks following the rainy season. 
Those plants with shallow absorbing root systems, therefore, 
are limited to a very short “absorbing season,” a season 
which represents in most cases but a small fraction of the 
year. 
It is obvious, therefore, that the continued life of suc- 
culents is not to be explained on the basis of a constant and 
uninterrupted absorption of water. The roots of these plants 
seem rather to be adapted for the prompt absorption of water 
at the time of the seasonal rains, and in this they are not 
infrequently aided by the rapid production during the wet 
season of additional absorbing roots—temporary roots which 
are shed as soon as the dry season approaches, much as 
leaves are. On the other hand, the success of the succulents 
in the desert is due in large part to the storage of large quan- 
tities of water, and to the careful guarding against exces- 
sive transpiration with consequent conservation of much of 
the absorbed and stored water. This economy is so effective 
that the plants are able to live from one rainy season to the 
next on the water gathered and stored during the short, 
rainy period. Indeed, there is evidence to show that this 
economy is more stringent than is actually necessary for the 
preservation of life, as plants have been observed to live for 
several years without absorption of water, whereas in the 
desert they usually have at least two periods of absorption 
during the year. 
As regards the storage of water, this is usually effected in 
extensive areas of tissue, composed of large water-storage 
cells. Succulency, therefore, is a term applied to plants pro- 
vided with a conspicuous amount of such water-storage 
tissue. This tissue may appear either in the stem or in the 
leaves, or in both, and as a result give rise to fleshy, or suc- 
culent, stems and leaves. In the typical desert cacti this 
tissue appears almost exclusively in the thick, fleshy stems, 
although in the century plants and other forms it is con- 
spicuously present in the leaves. The development of these 
water reservoirs, however, does not alone insure a constant 
water supply to the plant. A necessary accompaniment is the 
prevention of excessive evaporation, or transpiration. This 
conservation is effected first of all by a reduction in the total 
