138 BOTANICAL GAZETTE [AUGUST 
grounds for assuming provisionally that a considerable proportion of 
these are still capable of serving this purpose. If they are thus 
active, their osmotic force, as measured by plasmolysis, is several 
times that of the younger cells nearer the root-tip. It is apparent, 
in any case, that the osmotic force exhibited by the root-hairs and 
epidermal cells that are indubitably active is amply sufficient to 
account for the capacity of this plant to absorb water from the soils 
in the vicinity of the Desert Laboratory, even after such periods of 
drouth as those of the present year. Their absorption, however, is 
necessarily limited by the amount of water available. This, as we 
have seen, is also a determining factor of transpiration. The means 
by which the latter is controlled will be discussed elsewhere. 
That the creosote bush is able, through its absorbing cells, to 
abstract continuously a certain amount of water, however small, from 
such dry soil as that of the desert mesa, to maintain transpiration 
through many months of excéssive drouth, and at the same time to 
regulate nicely the amount of transpiration to correspond with avail 
able water supply, while all the time it is capable of living and does 
live as an ordinary mesophyte when given a suitable supply of water, 
is a remarkable fact. Its explanation involves more perfect knowl- 
edge not only of the physiological habits now under investigation, but 
also of the geographical history of the species, which still remains 1 
be written. It need hardly be said that the data for both are to be 
sought first of all in the desert where this plant is at home. 
I desire to express my sincere thanks to Dr. W. A. Cannon, the 
resident investigator of the Desert Laboratory, and to Messrs. Coville 
and MacDougal of the Advisory Board for the admirable facilities 
that have freely been placed at my disposal. 
Desert BoTaNIcaL LABORATORY, 
ucson, Arizona. 
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