NATIVE WOODY PLANTS OF THE UNITED STATES 15 



of fertilization of the pistil in the same flower or even of those on 

 the same plant. An instance of such a condition is found in the wild 

 plums where one plant often gives rise vegetatively to a colony. The 

 pollen in the flowers of the whole colony may be unable to effect 

 fertilization, and it is thus necessary in planting for wildlife food, 

 to ensure that stock planted is collected from at least several different 

 colonies and well mixed before using. According to Paul B. Sears, 

 who has experimented with plums, it is nearly always possible to 

 induce better fruit production in one colony by introducing a few 

 plants from a neighboring colony, or even by throwing flowering 

 branches from a different colony into the first. 



Production of fruit depends also on such environmental factors as 

 temperature or cloudiness and precipitation during flowering. These 

 conditions may very often be local, and certainly account in part for 

 the patchy distribution of fruit production of the same species. Also, 

 many plants that produce fruit freely in the sun will produce very 

 little in the shade, as Corylus, JSymphoricarpos, and Parthenocissus. 

 For this reason where fruit for wildlife is desired, a variety of species, 

 blooming at different times, is necessary for ultimate production. 



THE CORRELATION OF SOILS AND PLANT GROWTH 



Many statements that a given species will grow on a certain kind 

 of soil will be found herein. These statements are based for the most 

 part on observations of plants in their native habitats, made by many 

 different workers. They are extremely general, the correlation of 

 vegetation with soils being by no means certain. Soil studies that 

 have been made deal with factors that are not directly related to 

 plants, or else the relation of plants to such factors is not yet clear 

 (363). 



The distribution of some plants appears to depend directly 011 

 certain edaphic characters. Many species are almost entirely con- 

 fined to acid or to alkaline soils. The near absence of legumes from 

 very acid soils may be attributed to the inability of their symbiotic 

 bacteria to succeed under acid conditions. Similarly, the ability of 

 certain fungi to succeed under varying degrees of hydrogen-ion con- 

 centration may condition the ecesis of woody plants on the roots of 

 which they form mycorrhiza. The presence of toxic substances, the 

 release of which may depend also on acidity, may prevent a plant 

 from growing on a soil that under different local conditions might 

 easily support that plant. The breakdown of litter, for example, is 

 not at all understood. For such reasons as these, and for many 

 otherSj we are far from having identified the factors in soil whose, 

 variations are responsible for the local variations in plant distribu- 

 tion. 



Certain physical factors of soil, however, are worth mentioning 

 here. Shallow soils on an impervious subsoil may endanger the sta- 

 bility of trees by restricting root growth. Heavy soils may act in the 

 same way. Soils of spring-fed swamps and northern slopes are gen- 

 erally colder than other soils, and sandy soils and soils of southern 

 slopes are generally warmer (573). Higher and lower soil tempera- 

 tures have respectively a stimulating and retarding effect on plant 

 metabolism. 



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