90 College of Forestry 
condition furnishes a pretty good criterion for measuring the 
general trend and the successive stages of vegetation. This 
factor is the quantity and condition of the water of (and in 
water habitats over) the substratum. On this basis, which 
is of course a standard and well known basis of classi- 
fication of vegetation in ecological botany, * the glaciated ter- 
rain would present various degrees of the moisture supply as 
it would affect plant growth, from habitually dry or arid 
habitats (bare rock, loose sand, fallen logs) to water-covered 
or water-soaked soils (excess of water) on the other extreme. 
Between these, soils of such structure and drainage conditions 
as to facilitate the holding of moisture in film form about 
soil particles as in well-tilled field soils. In spite of the 
diverse conditions of the substratum and the forbidding 
aspect of some of it, it would seem as if there were no 
feature of it from which plants, at least of some sort, were 
wholly excluded. Dryness, submergence, heat and_ cold, 
even wind and water erosion are not absolute barriers in 
this State. For green plants the one absolute limiting 
factor would be light and this would apply practically only 
in limiting the depth to which green plants would invade 
lake bottoms. We are especially concerned with the de- 
velopment of vegetation from the two extreme conditions of 
the moisture relation: Namely (1) Upon a substratum 
under water or with excess of water. (2) Upon a sub- 
stratum normally deficient in moisture. 
The Developmental Sequence of Vegetation upon a Sub- 
stratum Having Excess of Water. 
1. Floating Vegetation of the Open Water. 
The history begins here with aspects of plant life which 
are not at all associated with the lake bottom or else merely 
lie upon it. They are not large plants anchored to the soil 
by roots. They embrace — 
(1) Pranxron.— The free-floating, microscopic life of 
lakes and ponds is to be considered in this connection as 
See under “ Ecology” in elementary textbooks on botany. Also 
2 3 2 De : : jae ’ 
Warming, (English translation) Ecological Plant Geography. 
