ORIGIN OF DESERT FLORAS. Laz 
the machine or its processes may materially modify their properties, or 
cause radical alterations in the processes themselves. 
These adjustments of function and alterations in structure are no more 
than the response of the individual, however, and are not transmitted 
to its descendants unchanged. The transfer of a plant from a warm to 
a colder climate, or from a moist to an arid region, may result in altera- 
tions which leave some effect when the plant is returned to its original 
habitat, lasting perhaps for two or three generations, but so far proof is 
lacking that any irreversible modification has been caused by the action 
of external conditions upon the soma or vegetative part of a plant. It 
would therefore be unsafe to assume that the desert flora owes the origin 
of any of its constituents to gradual adaptations resulting from the 
action of these conditions. 
Another aspect of the subject remains to be considered. In addition 
to the direct physiological, somatic response of a plant expressed in its 
roots, stems, or leaves, it is to be borne in mind that many of the agen- 
cies which cause these variations also act directly upon the germ-plasm, 
which gives a direct response, and not of an adaptive nature. 
By the controlled application of climatic factors, Tower has been able 
to induce the appearance of strains of beetles which diverge from the 
parental type in one main character, with correlated variability in others, 
which crossed readily with the parent and might be swamped by it. 
By the application of reagents to the reproductive elements of seed- 
plants I have been able to cause the appearance of new forms of plants 
which diverge from the parent in several qualities, which do not readily 
cross with the parent, and the newly acquired characters are irreversible 
so far as the tests of three generations may be taken as conclusive. Re- 
sults of a similar nature have been obtained by Gager with the use of 
radium preparations. The direct effects might consist in a modification 
of the relative speed or precession of the various processes in the gamete, 
or of the activities of the chain of enzymes, or of some disturbances of 
the delicate balance among the ions in the protoplasm. Still beyond 
these, stimulative responses or reactions may occur, which would find 
their ultimate expression in deviations in external form of the progeny 
of an individual subjected to the unusual agencies in question. Changes 
of the same nature may well be induced by many conditions to which 
plants are naturally subject, the survival of the resultant forms being 
solely a matter of possible interbreeding and habital selection. 
- A stream takes its rise near the alpine plantation of the Desert Labo- 
ratory and flows out on the desert a few miles away and a mile lower 
down. Doubtless hundreds and thousands of seeds are carried to the 
lowlands each year. Some of these develop into individuals which carry 
out reproduction. This is usually done in the native habitat, at actual 
temperatures of the tissues not above 60° or 70° F. Down below, spore 
