Hill: Penobscot Vegetation 355 



These plants tend to furnish an obstruction to the water 

 and permit the deposition of sand and gravel, and gradually 

 the soil among the boulders is built up higher and higher 

 until shrubs and other herbaceous forms gain a foothold. 

 Among these may be noted : 



Scirpus atrovirens Thalictrum polygamum 



Carex stipata Spiraea latifolia 



Salix cord at a Apios tnberosa 



Betula populifolia Mimulus ringens 



Alnus incana Aster umbellatus 



In one case a small alluvial island has been built up on the 

 plain and this is entirely covered with a dense growth of 

 Calamagrostis canadensis (Figure 15). 



3. Formations of Well-drained Uplands along the 



Seacoast 

 The character of vegetation along the seacoast varies not 

 only with the substratum, but with the degree of exposure as 

 well. The possible seaward limit of vegetation is deter- 

 mined by the upper level of effective wave and ice action. 

 In protected situations plants may extend to the water's 

 edge, thus occupying all the area in which it is possible for 

 them to develop. Succession on both rock outcrops and un- 

 compacted rock in such places is entirely similar to that al- 

 ready described for ordinary uplands, and in the absence 

 of unfavorable factors there may occur a complete develop- 

 ment of the climax forest. On level rocky surfaces there is 

 usually a sharp transition between the forest and the water. 

 Occasionally, however, the rocks may drop off abruptly, 

 forming miniature cliffs. When shaded by overhanging 

 foliage the surface of these rocks may be cool and moist and 

 support an abundant growth of bryophytes, such as species 

 of Philonotis, Dicrannm, Mnium, and Scapania, together 

 with herbaceous forms like Viola pattens, Sagina procum- 

 bens, Circaea alpina, Ilex verticillata and several willows 

 and alders. In drier, sunnier places the vegetation may 



