484 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



silt deposited on the leaves and flowers the plants looked no worse 

 for having been submerged. Nonetheless, even a week of arrested 

 development during the growing season could be crucial in preventing 

 seed development. 



"Wind, desiccating in summer and freezing in winter, is a relentless 

 force in the arctic-alpine environment, influencing both the form and 

 function of the plants. Tender young shoots have a veritable gantlet 

 to run in order to become established, especially if they project any 

 distance at all above the protection of the surrounding vegetation. 

 At timberline, the low trees and shrubs often betray this ceaseless 

 pounding of gusts and gales by pointing all their limbs in the direction 

 of the prevailing winds (pi. 8), and arborescent growth here is in gen- 

 eral likely to be gnarled and dwarfed from winds (pi. 8). Instead of 

 growing vertically, the plants, plied by the winds during their tender 

 years, sprawl horizontally over the meadows and screes. This prostrate 

 twisted habit, typical of tree limit anywhere, is sometimes called 

 Krummholz after the Germans. Beyond tree limit all growth, her- 

 baceous or woody, is dwarfed except in sheltered spots, and we know 

 from horticultural and ecological research and experience that the 

 dwarf habit of many arctic-alpine species is at least in part genetically 

 controlled. This habit surely must be an adaptive response in large 

 part to the factor of wind. I must hasten to append the caution, how- 

 ever, that constant strong winds are not universal in the Arctic, and 

 therefore the dwarf habit cannot be so easily explained as to attribute 

 it simply to wind. 



Everyone knows that the Arctic is the land of midnight sun. Indeed, 

 to the plant the annual light regime is very bizarre. Each summer for 

 several weeks or more the days last for 24 hours and light is continuous. 

 In view of the short summer, it is doubtful that any vascular plant 

 could propagate itself by seed if it were not for this additional daily 

 insolation. In large part the longer days compensate for the shorter 

 season. Light intensity is not uniform throughout the 24-hour day, 

 of course, but falls off sharply at night. The nights are often like an 

 extended early twilight or early dawn. Owing to the more oblique 

 angle of incidence throughout the higher latitudes in summer, the 

 sun's rays must pass through a thicker layer of atmosphere than in 

 temperate and tropical regions, and they strike the land surface with 

 less intensity, hence less heating capacity. As one might guess, the 

 longer duration, lower intensity, and somewhat different quality of the 

 light have definite effects on the plants. Quite a few arctic plants are 

 "long-day" plants ; this means that they require relatively long days 

 and short nights for flowering and cannot be brought into flowering at 

 lower latitude without supplementary light. Kosette-f orming species 

 are very common in the Arctic, and there is some evidence to suggest 



