SUCCESSIONS IN WATER 229 



lichens because of protection from the sun and the constant presence 

 of moisture. Where the rocks are exposed to the sun, however, xeric 

 lichens are the pioneers. In general, the succession is more or less 

 comparable to that on a vertical rock cliff as the canyon broadens 

 out and conditions become less severe. Successions in rock canyons 

 have not been sufficiently studied to enable us to know accurately 

 just what does happen. Such canyons, therefore, offer a fertile 

 field for investigation. 



138. Successions in Water.— Successions may take place either 

 in standing water or in running water. The successions in standing 

 water have been more thoroughly studied than any others, due to the 

 fact that often all stages can be found forming distinct zones 

 around a single body of water. 



In the normal succession in a depression that is not entirely unpro- 

 vided with drainage, the pioneer plants are floating or submersed 

 aquatics that grow in water 5 to 10 feet in depth. Characteristic 

 plants are many kinds of algae, the duckweeds (Lemna), the pond 

 weeds (Potamogeton) , etc. As the depression becomes filled and the 

 water more shallow, 2 to 5 feet deep, plants of the second stage, which 

 root at the bottom but have shoots which extend nearly to the top 

 of the water, or float, in part, upon its surface, come in. This stage 

 is characterized by water lilies (Nymphea) and such plants as the 

 water crowfoot {Ranunculus aquatilis) and the water smartweed 

 {Polygonum amphibium). In still shallower water, up to 2 feet, 

 we find the bulrush-cat-tail stage characterized by such plants as 

 the bulrush {Scirpus), the cat-tail {Typha latifolid), the pickerel 

 weed {Pontederia cordata), the arrow leaf {Sagittaria latifolia), etc., 

 whose shoots extend well above the water. 



After the depression has filled to such an extent that the water 

 table is just at the surface there appears a stage which is char- 

 acterized by various species of sedges and grasses and such flowering 

 herbs as wild iris {Iris versicolor), skull-cap {Scutellaria), water hore- 

 hound {Lycopus), and several kinds of gentians {Gentiana). This 

 stage is followed by the shrub stage in which the dominant plants are 

 likely to be willows {Salix), alders {Alnus), button-bush {Cephalan- 

 thus occidentalis. and dogwoods {Cornus). The first trees to follow 

 the shrubs are likely to be ashes, soft maples and elms. These are 

 replaced later, as the substratum becomes drier, by the trees of the 

 climax forest. 



When the depression is poorly, or not at all, drained the normal 

 aquatic succession is modified into a bog succession. In this the first 



