HARPER: PLANT POPULATION OF MICHIGAN dl 
Furthermore, most marsh plants have hollow or spongy stems or 
petioles, or aerenchyma, or pneumatophores, all of which doubt- 
less serve to conduct air to the roots. 
It might be observed here, parenthetically, that many plants 
which in the region under consideration are chiefly confined to 
bogs grow equally well on uplands in colder climates.* This is 
probably because the low temperature and short growing season 
farther north so limit the availability of the nutrients in the soil 
that none but slow-growing plants can thrive. 
An elaborate system of hypotheses of succession has been 
postulated by Gates and others who have worked in this or some- 
what similar regions, and some have even gone so far as to try to 
connect all the plant associations in a limited area by successional 
relations. But some of the imagined successions can never take 
place without profound topographic changes, which may or may 
not come to pass, and with which the botanist is not particularly 
concerned. There are, however, two genuine types of succession 
(biotic succession as defined by Cowles,} and distinguished from 
his regional and topographic successions) which can be studied to 
advantage in this region. The first is that connected with the 
filling of lakes, etc., with vegetation and the gradual accumulation 
of peat and humus. In a coniferous swamp the falling leaves, 
twigs, trunks, etc., gradually pile up high enough above the ground- 
water level to be subject to ordinary decay, and thus form humus 
or duff instead of true peat. In such humus grow many plants 
which are equally characteristic of the upland hardwood forests, 
and this has led some to believe that the swamps, barring human 
interference and unforeseen complications, will ultimately be 
replaced by beech-maple-hemlock forests. But there are quite a 
number of plants in this region which seem to demand both humus 
and access to mineral soil or alkaline peat, such as Acer saccharum, 
Tsuga, Fagus, Tilia, Ulmus, Quercus alba, Viburnum acerifolium, 
Vagnera racemosa, Carex arctata, C. laxiflora, Washingtonia, Actaea 
alba, Circaea, Adiantum pedatum, Geranium Robertianum, Caulo- 
phyllum, and several others less common here, and we have no 
* See Livingston, Bot. Gaz. 39: 40. 1905; Harper, Pop. Sci. Monthly 85: 340. 
I9r4. 
+ Bot. Gaz. 51: 161-183. I9gII. 
