372 Spalding: Absorption of atmospheric moisture 



It appears, then, in the case of Foitqideria, that (i) the leaves do 

 not, even under favorable conditions, absorb water vapor from the 

 atmosphere ; (2) the branches are capable of such absorption, as are 

 also the buds, though the latter "are not more specially concerned 

 in the process than the former ; (3) both shoots and leaves give 

 off water rapidly, not only in a dry atmosphere but also in one of 

 rather high relative humidity ; (4) as is well known, this process 

 is checked or practically suspended in an atmosphere nearly or 



quite saturated with water vapor. 



Herein, apparently, lies the explanation of the fact that in spite 

 of the very scanty rainfall of the summer, the ocotillo has remained 

 in leaf during so large a part of the present year. It is not due to 

 absorption of water vapor from the atmosphere, although the rela- 

 tive humidity, during the period of observation, has reached the 

 extraordinary average of approximately 60 per cent., but it is due 

 to the fact that water, which with lower humidity would be given 

 off, has been retained by the plant. While, too, leaf-formation 

 may be induced, as shown by Lloyd, by artificial irrigation of the 

 stems, but not, as far as the present study indicates, by absorption 

 of water vapor from the atmosphere, there results, nevertheless, 

 from a long period of high relative humidity what is apparently of 

 greater advantage to the plant, namely, the retention, for months 

 in succession, of leaves already formed and in condition for produc- 

 tive work. Had the plants been defoliated, as they must have 

 been if lack of rain had been accompanied, as usual, by low rela- 

 tive humidity, even the maximum number of new leaf-coverings, 

 reported by Cannon, if they could have been produced, would 

 probably fall far below the single continuous leaf-covering in 

 productive energy, though this is a matter the settlement of which 

 would involve another and very different line of experimentation. 



Experiments similar to those already described were next car- 

 ried out with Ccltis pallida, which was remarkably fresh and green, 

 notwithstanding the slight rainfall of the preceding summer. The 

 leaves were found to lose weight rapidly in a dry atmosphere 

 upwards of 24 per cent, in three hours — and to gain extremely 

 slowly in an atmosphere nearly saturated with water vapor — only 

 eight-tenths to nine-tenths of one per cent, in five hours. Later, 

 after abundant rain, fresh ends of branches, with leaves, lost 31 



