108 
TENTH REPORT. 
(2) Plants of but few types, and of simplest structure, are able to live 
wholly submerged in water, and the number becomes rapidly less as the 
water becomes deeper, until it is rare to find any species which is perman- 
ently attached by roots to the soil, growing from a depth greater than 15 
feet below the water surface, the maximum depth from which they start 
apparently depending upon the transparency of the water, although doubt- 
less, other factors of growth must be taken into account. The floating Algae 
and the few seed plants which have no root attachments are not significant. 
(3) Aquatic seed plants which have a part of their stems and leaves floating 
or emerged, grow in shallower water than the wholly submerged types, and 
become abundant in less than 10 feet of water, and, in the case of those with 
erect stems, rising much above the surface, 5 feet is about the maximum 
depth at which any are able to persist, while less than 2 feet limits the greater 
number of species. In water of less than this depth, a considerable number 
of species may give character to the vegetation. 
(4) The turf, or mat-forming types, sedges and others, are limited lake- 
ward by a depth of water somewhat more than one foot, and will persist for 
a time to about the same height above water level; as these plants build the 
deposit above the water level, and prepare the way for the woody plants, 
their relations are important. 
(5) As soon as the surface of a peat deposit is above water, a great variety 
of marsh plants, capable of enduring a good deal of water about their roots, 
take possession of it and change the character of the material making up 
the peat. Most characteristic, and usually appearing first, are the shrubs, 
especially the bog heaths and certain willows. Closely follow.ng these types 
come the coniferous trees, and more often, Sphagnum. 
(6) It is apparent that this series is only to be found complete in depres- 
sions permanently filled with water. It is also evident that where moisture 
conditions are favorable, a deposit may begin with any member of the series — 
thus on a wet, poorly-drained land surface, in a moist climate, the peat may 
be built up wholly from a single association of plants. 
(7) There are evidently two general kinds of peat deposits recognizable 
from the physiographic form of the surface, the filled depression and the 
covered flat surface, or the “ filled” bog and the “built-up” bog. 
Whatever may be the order in which these items are taken up, it is ap- 
parent that two lines of investigation must be pursued rather thoroughly, 
if the full significance of the peat is to be understood. 
Perhaps the most readily obtained information to be derived from this 
class of deposits, is that regarding climatic conditions and changes during 
the period in which the peat has been forming. In the north of Europe, 
notably in Denmark, and Scotland, examinations have been made which 
show that in the case of certain peat deposits, there have been cycles of 
milder climatic conditions, followed, and preceded by more severe ones, as 
certain beds of peat contain the remains of southern plant species which 
are associated above and below with' those of more northern types. In 
our own country but little is known of the records to be found in the beds 
of Post-glacial peat, so numerous in the glaciated regions of the continent. 
In a few cases, the writer has found definite records of prolonged drought 
followed by wet periods, which must have been of considerable duration, 
but it is only under rather unusual conditions that it would be clear that a rise 
of water indicated by a complete change in the character of the plant remains, 
was due to increased rainfall, and not to interference with the drainage. It 
