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 
]Dlums 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 Gorylus^ SymphoHcarpos^ 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 toi 
plants, or else the relation of plants to such factors is not yet clear 
{363). 
The distribution of some plants appears to depend directly on 
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, thei 
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 break-down of litter, for example, is 
not at all understood. For such reasons as these, and for many 
others, we are far from having identified the factors in soil whosci 
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. 
130186°— 39 2 
