THE PLANT WORLD 199 



bottom is lieaved up into a low lying island, which is scarcely above the 

 water at ordinary stages and is frequently submerged during the rising 

 stages, only to be left a mud flat as the receding period comes. Islands 

 built by pure river agencies are seldom very large, but another agency 

 by way of vegetation now comes and aids very materially. 



The island has scarcely emerged from the river before seeds borne 

 hj the currents or on the wind fall upon it, strike root, and start out to 

 run their life course. The common black willow, Salix nigra, and also 

 S. longifoUa, are present in large numbers and are soon crowding, so 

 much so that many perish in the struggle, but the vigorous thrive. The 

 roots readily reach the water level in the soil, the rising and subsiding 

 waters bring rich soil coverings and the problem of life becomes easy 

 for the embryo forest. Greater stability as a result comes to the island. 

 The current is checked above, deflected to either side and shoals are 

 formed along the island and immediately below, and new additions are 

 constantly made to the land area, which in turn receive their contingent 

 of willow seeds and subsequent trees. The older portions of the island, 

 however, do not remain the same. High water has caused a more rapid 

 deposit of sediment on the interior of the island because of the checking 

 of the current occasioned by the obstructions due to willow growth, and 

 in low stages the dry sand near the shore is carried inland by the wind 

 and deposited in long narrow low dunes which are scarcely perceptible 

 in the early stages of the island growth. The seeds of Salix discolor, 

 finding conditions favorable, begin and develop rapidly and displace the 

 I)re\dous growth of Salix nigra and S. longifoUa, which prefer the condi- 

 tions near the shore and constantly migrate as the island grows. As 

 the interior of the island develops by the agencies of wind and high 

 water, the cottonwood, Populus monilifera (P. deltoidea), appears and 

 readily makes headway against the Salix discolor zone, which in turn 

 takes the Salix nigra zone and all make for themselves distinctive zones 

 in new positions. The island as viewed from off shore presents in this 

 stage of its development three distinct zones of tree growth, namely, be- 

 ginning at the shore line: 



1. Black willow zone (Salix nigra). 



2. Glaucous willow zone (S. discolor). 



3. Cottonwood zone (Fopidus monilifera). 



To these we may add a fourth zone, viz.: the elm zone. The white 

 elm seeds are readily carried by the wind or the currents to the island 

 shores, but the soil is too damp for healthy growth, although a few 

 small individuals may be found among the Salix discolor or Populus 

 monilifera zone. As the interior of the island develops the higher por- 

 tions become dry enough to come within the limits of the life range of 

 the white elm, and individuals are to be found ranging in time upwards 



