412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



AERIAL ALGAE 



Undoubtedly the algae with which we are all most familiar are the 

 aerial algae or those algae that obtain their water wholly or in large 

 part from the moistm'e in the air. Strictly aerial algae are found 

 on the bark and leaves of trees, on woodwork, stones, and rocky cliffs. 

 Since atmospheric and hygrometric conditions are of vital importance 

 to these algae, they must be able to survive long periods of drought 

 and recover quickly when given the opportunity to absorb water again. 

 Certain forms have demonstrated their ability to resist drought for 

 continued long periods and have survived in a desiccator for 6 months, 

 then absorbed water rapidly in their dry condition. In districts where 

 severe storms are frequent, aerial algae are not numerous, as they 

 are washed away from exposed woodwork and brickwork. However, 

 they may be found on the ground or lower portions of the woodwork 

 and brickwork where they are protected. Aerial algae usually grow 

 on the shaded side of the support or substrate, but protection from 

 the prevailing winds is probably of greater importance than shade. 

 Certain species of the aerial alga Prasiola live only where the sub- 

 strate is rich in nitrogen. They grow luxuriantly in rookeries and 

 on rocks or cliffs, where they live on the excrement of birds. 



Nature is the perfect artist when left to her own devices. In her 

 ardor for beauty she cunningly shrouds old broken-down fence posts, 

 horse troughs, well and cistern coverings, wooden turbine conduits, 

 ditches, roofs, and the detritus of barnyards with the restful and 

 beautiful colors of the algae. In special honor to the whalers and 

 other sea lovers buried in the old graveyards on the eastern coast 

 of New England, she wreathes their dark gi-anite tombstones that 

 are cracked and almost ready to crumble with graceful masses of 

 golden algae. This same alga, which, though a green alga, may 

 af)pear with different pigmentation in accordance with the nutrient 

 solution or light in which it is growing as shown by our experiments 

 here, covers the red roof tiles of a hut, or sparse soil, and beech 

 trees with bright yellow mats of luxuriant growth. It is a greener 

 color in the shade, and in damp weather it is unusually conspicuous 

 against its dark backgrounds, while on dry days it adheres closely 

 to the surface of attachment. In India, the aerophilous algae 

 produce alternating red, green, and black bands on the bark of 

 Oreodoxa regia. The red coloration is due to Trentepohlia umbrina, 

 the green to Protococcus viridis, and the black to Scytonema ocel- 

 latum. Another species of Trentepohlia is responsible for the rusty 

 coloration of cement and masonry walls. 



The Pedras Negras in Pungo Andongo in Portuguese West Africa 

 receive their names from an alga that grows in black stripes on the 



