1905] CANNON—WATER-CONDUCTING SYSTEMS 399 
differences in the character of the soil, is shown by the manner in 
which they react to an abundance of water when they are well irri- 
gated, the substratum being otherwise the same. The creosote 
bush and both species of palo verde thrive equally well by the side 
of the same irrigating ditch, and the same is true of the other plants 
mentioned above. That is to say, each of these plants, whatever 
may be their natural habitat, is most vigorous where the water supply 
is adequate. The suggestion cannot be avoided, therefore, that 
the distribution of these plants in this vicinity is in some way indicative 
of their specific reaction to the minimum rather than to the optimum 
supply of water. The extent to which this phase of the reaction of 
desert plants to water may influence their present distribution in 
this vicinity will be the subject of future experimental investigation 
at the Laboratory. 
As has been suggested above, the irrigated desert plants are the 
more vigorous in every way. The leaves are larger and are functional 
for a longer period, and the plants attain also much larger proportions 
than those that are not irrigated, and they also grow faster. To give 
but a single instance of the latter, a palo verde well-irrigated four 
years from the seed is now about eight feet high, while there are many 
of these trees in the neighborhood of the Laboratory, without doubt 
Many times as old, that are not so high. 
With the larger leaf surface of the irrigated plants one would 
look for a faster rate as well as for a larger total transpiration. What- 
ever may be the absolute transpiration of the irrigated forms, the 
Structure of the stems of the irrigated as opposed to the non-irrigated 
plants indicates that the latter have the faster rate. That is, the non- 
irrigated stems have larger ducts and more of them per equivalent 
area of cross section than the irrigated stems, which can be taken 
as indicating the more rapid transpiration of the former.’ 
To anticipate the general conclusions to be drawn from this 
Paper, we may therefore be justified in thinking that the desert 
plants of this locality, being probably better provided with conductive 
elements than those of the Egyptian-Arabian deserts, may also have 
a faster rate of transpiration. 
3 inci Physiology of plants 1: p. 216; HABERLANDT, Pflanzenanatomie 
(3d ed.), p. 
