9 o 



BOTANICAL GAZETTE [February 



to vegetation as the amount of direct solar radiation and radiation of 

 heat from the ground. The value of the former factor in climatology 

 has long been recognized by meteorologists. 



We must never lose sight of the fact, however, that it is not one 

 factor alone that determines the development and distribution of 

 the vegetation, but a resultant of the many different conditions to 

 which the plants are subjected. This has been duly emphasized b} 



Warming (14). 



Light. — In the closest relation to atmospheric temperature, and 

 especially to radiation of heat, is the factor of light. The intensity of 

 illumination is remarkably large on the open sand formations of the 

 coast, and this circumstance is noticeable in the many protective adap- 

 tations of plant structures against the influence of light. There are 

 as yet no reliable means of ascertaining the intensity of the light, and 

 we have consequently no basis for comparisons on this subject. The 

 strong insolation on the white surface of the sand favors a greater varia- 

 tion of temperatures than on other formations of the coast. The radi- 

 ation is, however, generally less on the coast than in localities far a^ay 

 from the sea because the larger quantity of aqueous vapor in the 

 atmosphere in the former place tends to check this terrestrial radiation. 



Hydrometeoric conditions.— This term has here been used to 

 distinguish the factors of atmospheric humidity from those of the water 

 content of the soil or the substratum from which the plants take their 

 supply. On coastal sands it is perhaps more apparent than on other 

 formations that there is some difference in influence and effect on 

 vegetation of the moisture contained in the air and of the water in the 

 soil. It is, however, always extremely difficult to decide to what extent 

 certain adaptations are due to one factor more than to another. 

 especially when we do not possess detailed observations. 



AlR MOISTURE AND EVAPORATION CAPACITY.— The supply °f 



atmospheric moisture is to a great extent obtained from the ocean 



* 



and it follows that on the coast the amount of humidity must be much 



greater than farther inland. 



know 



ryday 



dense 



\ 



know 



that moisture in the air greatly lessens transpiration, and, other factors 

 being equal, transpiration ought therefore to be less on sea shores than 



