Jan., 1922] UPHOF PLANTS IN SOUTHEASTERN MISSOURI 
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especially in deep swamps; they grow successfully only when moisture is 
abundant. Taxodium is "the" tree for alluvial swamps. Lands which 
are overflowed regularly by rivers are especially suitable for this species; 
only in the dry seasons do a part of these forested mud fields become dry. 
The trees develop a straight stem; where water is deep, the base of these 
stems is always broad and more or less ridged ; where w^ater is more or less 
shallow, or in localities not as much subject to heavy overflow, this base is 
not as broad or ridged and is normally developed like that of ordinary trees. 
A peculiar characteristic of these trees consists of the development of knees, 
Fig. 2, A swamp forest. A, Nyssa sylvatica. B, Salix nigra. C, Acer ruhrnin, Quercus 
alba, Q. rubra, Platanus occidentalis , etc. D, Cephalanthus occiden talis. E, Taxodium 
distichum. 
vertical outgrowths of the superficial root system, which give a strange 
aspect to the physiognomy of this plant association. Knees occur only 
where the ground is covered for a long time or all the year round by water. 
These knees attain a height of from 4 to 15 decimeters ; very occasionally they 
are higher. According to various authors they serve the purpose of supply- 
ing oxygen to the root system in the mud, as well as strengthening the 
support of the tree on the surface. 
Another representative of the swamps which is frequently associated 
with Taxodium is Nyssa aquatica L. A pure forest tree of medium age 
shows a peculiar aspect on account of the straight, smooth stems ; hundreds 
of such trees have the appearance of as many straight pillars. Old in- 
dividuals attain a height of from 18 to 28 meters. The base of old stems 
is thickly covered by mosses and by a smdWiem, Poly podium poly podioides 
(L.) Hitchc. Where Taxodium swamps are not too deep there exists an 
immense undergrowth of Polygonum Muhlenbergii (Meisn.) Wats. On 
outstanding mud flats or in open shallow places of the Taxodium forest, 
Cephalanthus occidentalis L. is of common occurrence, forming dense thickets. 
Sometimes a single Salix nigra Marsh., or rarely Dirca palustris L., occurs; 
in some swamps the author observed a dense growth of Leitneria floridana 
Chapm. in places similar to those occupied by Cephalanthus, while on 
logs Ilea virginica L. often occurs. 
Where the swamp becomes shallow, or where it is even dry during a 
large part of the season, a greater variation in species of trees and shrubs at 
