54 
THIRTEENTH REPORT. 
an explanation of this, we are confronted by difficulties which can only 
be solved by a careful study of conditions within the plant. The harmful 
effect might be due to an increased concentration of the juices of a plant; 
or possibly changes in permeability; or to an actual harmful effect on 
the cytoplasm; or to an interference with the working of an enzyme or 
some other process. The presence of certain salts in solution may either 
accelerate or greatly retard the action of enzymes and it is very probable 
that in plants as in animals, the presence of certain salts is useful in 
accelerating or retarding various processes. Unfortunately, however, 
we know entirely too little about the condition of substances within the 
plant. A good example of the importance of changes in the relation 
between conditions in the soil and in the plant would seem to be afforded 
by the non-available water content of the soil. During the day, the 
leaves of plants are manufacturing sugars which increase the concentra- 
tion and osmotic pressure of the solution in the cells. At night these 
sugars are used or removed and the concentration and osmotic pressure 
again decreases. Since water is probably drawn into the cells by osmosis, 
we would naturally expect that in the middle of a bright day when the 
osmotic pressure is high, the water would be drawn into and held in 
the cells of the leaf- with greater force than during the night or early 
morning, or on a dark day, and that therefore, the amount of water in 
the soil which is not available to the plant would vary with the time of 
day and kind of day. This has proved to be the case to such an extent 
that these factors will have to be taken into consideration in all accurate 
determinations of non-available soil moisture. 
Enough has probably been said to point out something of the impor- 
tance of studying the conditions within the plant in their relations to 
soil problems. A thorough understanding of this relation would cer- 
tainly furnish a satisfactory way of handling many practical questions. 
The influence of the plant on the soil, like that of the soil to the plant 
would appear to be a fruitful subject for investigation rather than a 
field in which a great deal has been accomplished. Substances which are 
toxic to plants may l>e formed in soils as the result of the growth of 
plants, but whether these are usually excretions from the plant or the 
result of bacterial decomposition of tissues thrown off front the plant, 
is uncertain. Many plants particularly fungi and insectivorous ones are 
known to secrete enzymes and other substances so it is entirely possible 
that plant excretions may play an important role in changing the chemi- 
cals of the soil. The growth of roots appears to be accomplished in some 
cases by a dissolving of mineral substances, but how much of this is due 
to things outside the plant, such as acids produced by the decay of tissues 
is also uncertain. The meaning of the oxidizing and reducing power of 
roots is likewise little understood. 
In conclusion the soil problems from the standpoint of the botanist 
would appear to me to be one on which comparatively little has been 
accomplished but which offers tremendous possibilities. 
Michigan Agricultural College. East Lansing, Mich., April, 1911. 
