68 AQUATIC CONDITIONS 
in late summer or early autumn. Smaller bodies of water are similar 
in this respect. 
I. LAW GOVERNING QUANTITY (47) 
Liebig’s Law of Minimum, as applied to plants, is stated as follows: 
‘““A plant requires a certain number of foodstuffs if it is to continue to 
live and grow, and each of these food substances must be present in a 
certain proportion. If one of them is absent, the plant will die; if one 
is present in a minimal proportion, the growth will also be minimal. 
This will be the case no matter how abundant the other foodstuffs may 
be. Thus the growth of a plant is dependent upon the amount of the 
foodstuff which is presented to it in minimal quantity” (47, p. 234). 
The amount of plankton is determined by the same law. All food sub- 
stances must be present in the correct proportions. The amount of 
plankton may be determined by one substance which is deficient in 
amount. 
2. AGE AND QUANTITY (6 and citations) 
In bodies of water with small outlet, the quantity of plant and animal 
life probably increases with the age of the water body. This is because 
the foodstuffs are washed in by the inflowing water, and because rooted 
plants absorb food from the soil in which they grow, and when they die 
and decay these foodstuffs are added to the water. Accordingly, the 
older the pond and the longer rooted vegetation has grown, the greater 
the quantity of life. This principle is illustrated by an age-series of 
ponds at the south end of Lake Michigan to be discussed in detail later. 
The numbers used indicate relative age. Ponds 1, 5, 7, 14, 30, 52, 89, 
and 5 were studied, but especially 1, 5, 7, and 14 (6). Tables VI-VIII 
give a summary of the results. 
TABLE VI 
SHOWING QUANTITATIVE RESULTS OF EXAMINATION OF FacTORS RELATED TO 
QUANTITY OF PLANKTON 
Ponp NuMBERS—AGE-SERIES 
| No. of 
x 7 ad! Collection 
Total carbonates in parts per million ....| 138.800 | 160.200 | 160.300 I 
CO; cies periiters =: ace eee oe 0.0 Be/h 2.7 2 
OxygenicG:c) per liter35 4s ose 6.28 3-47 Deo 4 
Bacteria pericice-<1 se ernie ere 779 2AGO! i 355° 2 
* Average of collections, April, May, June, July, taken over sandy bottom (pond r) or at the top 
of submerged vegetation (ponds 7 and 14). 
