456 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
can not successfully occupy the same habitat if their capacity for 
root absorption differs in the same ratio. But supposing that with 
infinite patience and with a reasonable approach to accuracy both 
sets of physiological data have been determined, we are still, quite 
possibly, entirely in the dark as to the real cause of the different 
behavior of the plants under investigation. It may be in their case 
that the whole matter of absorption, conduction, and transpiration 
is beside the mark, and that certain plants can not succeed in the 
desert because the intense insolation exerts directly a prejudicial 
influence to which they have not become inured. The intricate 
nature of the subject is apparent, and it is also evident that there is 
little encouragement for any one to take it up who has not had 
extended training and thorough equipment for physiological re- 
search. Yet with all its difficulties the problem is an attractive one, 
and the abundance of material to be had in any desert city, together 
with the great mass of data that has accumulated in the hands of 
horticulturists and at the experiment stations, offers the best of 
opportunities for extended and fruitful work. 
If, as we have seen, the different deportment in the desert of plants 
growing, or having the opportunity to grow, side by side in well- 
watered ground, is an exceedingly complicated matter, by how much 
are the difficulties increased when we pass from a habitat of uniform 
and highly favorable conditions to the various and often extremely 
trying conditions which prevail in different neighboring habitats, 
such as the dry slopes underlaid by caliche, the salt spots, and others. 
Tf the case of a plant growing in well-watered soil may become des- 
perate because of the scorching winds or the intense insolation to 
which its top is exposed, what hope is there for one that essays to 
grow where both dry air and dry soil present the supreme test of 
endurance? As a matter of fact only relatively few species meet 
the test successfully, yet there are some that do, and they present 
some of the most instructive data yet derived from the study of 
desert plants. | 
But little reflection is needed to arrive at the conclusion that the 
classical question regarding the relative importance of physical con- 
stitution and chemical composition of the substratum to plant 
growth—though like the poor it promises to be always with us—does 
not and can-not reach the heart of the problem. For every plant 
which successfully holds its place in a true desert habitat there is a 
delicate balancing of the regulation of transpiration, the power of 
absorption, the capacity of the conducting system, the presence or 
absence of storage tissues, and, we may well believe, the possession of 
protoplasmic properties which contribute to its powers of endurance. 
This being the case, it would seem that in future, investigations of the 
habitat relations, of desert species especially, must be directed mainly 
