174 



THE PLANT LIFE OF MARYLAND 



ent. A plant which may occur in considerable numbers on the drier 

 part of a margin, for example, behind the Typha, is Phragmites 

 phragmites; rather large groups of which were seen on the Patapsco 

 Kiver near Baltimore. (Plate XIV., Fig. 2.) The zone in the 

 foreground consists of Ambrosia trifida. The extent of such marshes 

 depends on several factors, (1) steepness of slope of the river bank, 

 and breadth of the flood-plain ; in the case of most of the rivers 

 of our district there is not a well-developed flood plain, but a com- 

 paratively narrow channel with gently sloping banks, hence the 

 area available for a marsh is limited, and the tree zone is soon 

 reached. (2) Rapidity of descent of the river channel, or in other 

 words rapidity of flow of the river: the part of a river which flows 

 through this district necessarily has a gentle slope, for the Pa- 

 tuxent is at Laurel less than 200 feet above sea-level, moreover 

 tidewater backs up the rivers for a great distance, thus periodic- 

 ally destroying the flow in the lower reaches. These conditions 

 are favorable for the deposit of sediment, which is the necessary 

 concomitant of marsh formation. (3) Nature and quantity of the 

 sediment brought down by the river, which depends partly on the 

 last factor, but also on the nature of the country through which 

 the river flows in its upper course. If sediment is very fine it may 

 be carried to the mouth of a river instead of deposited along its 

 channel. In most cases a mixed condition prevails, and the sedi- 

 ment is sorted out as it flows along. Marshes of the type here con- 

 sidered are partly the result and partly the cause of the silting up 

 of rivers. After a storm much sediment is carried down a river, 

 and as the water of the river resumes its normal level some of the 

 sediment is deposited along the river's course, especially along the 

 margin, where the current is slow. The water is thereby made 

 shallow enough to permit the growth of such plants as Pontederia 

 and Sagittaria; continued growth of these plants not only holds 

 the sediment in place but helps to accumulate more. So the river 

 becomes shallower and narrower, and as a consequence the navigable 

 portion is restricted. The gradual raising of the level of the marsh 

 is effected partly by the detritus deposited on the vegetation by 

 spring freshets, and partly by the accumulation of dead plant re- 



