238 TEXTBOOK OF BOTANY 



climates are relatively modem. Previous to these discoveries every human 

 community was largely dependent upon local conditions. The discovery 

 of buried cities representing the former presence of large populations of 

 people in regions that are now deserts is historical evidence that trans- 

 piration is a primary factor in the restriction and migration of human 

 populations. At the present time more than 25 per cent of the land sur- 

 face of the earth is too deficient in moisture for crop plants without 

 irrigation, and only a very small portion of this land can be profitably 

 irrigated. 



Distribution of local plant communities. The differences among the 

 native plant communities that develop in ponds, marshes, shrub swamps, 

 swamp forests, and the adjoining upland are so marked that almost 

 everyone can recall the distinctive appearance of both the plants and 

 the communities. 



In the pond are wholly submerged "pond weeds," some rooted and 

 others not. On the surface may be floating algae or duckweeds, and 

 extending above the water surface are water plantain, arrowhead, water 

 smartweed, and many others. The marshes are characterized by bul- 

 rushes, cattails, sedges, and grasses; the shrub swamps bv certain dog- 

 woods, buttonbush, alders, and shrubby willows. The swamp forest is 

 dominated by species of sycamore, elm, ash, maple, hickory, and oak. 

 There are also scattered small trees and shrubs beneath the forest 

 canopy, and certain characteristic herbaceous plants form a ground 

 cover. An undisturbed upland forest in the same region consists of 

 another group of tree species such as the upland oaks, hickories, maples, 

 beech, walnut, and linden. It also has several layers of undergrowth — 

 small trees, shrubs, and herbs. 



In the chapters on photosynthesis attention was called to the manner 

 by which certain kinds of plants may exclude others in a forest by 

 overshading them. Likewise the discussion of respiration included the 

 fact that only certain species of plants can grow submerged in water, 

 or where the soil is saturated with water and deficient in free oxygen. 



Transpiration is a third factor in the elimination of species, and is 

 most effective in upland forests. Here high transpiration rates may cause 

 wilting and death during periods of drought. Again the plants most 

 affected are the seedlings with their limited root systems. But in ver\' 

 prolonged droughts certain species of trees in the swamp forest may be 

 injured to a greater extent than the upland trees because thev have 

 shallow root systems. In the complex forest communities the largest 



