ioo6 PRINCIPLES OF STRATIGRAPHY 



1. Hydropliytcs and HetnihydropJiytcs. Aquatic plants are 

 marine, or brackish, or fresh water. The marine types comprise 

 the grass-wracks or eel-grasses (Zostera marina, and Z. nana). A 

 few plants are confined to a brackish-water habitat {Ruppia mari- 

 thna, etc.). while others are adapted to both fresh and brackish 

 water. The aquatic vegetation of ponds and lakes may be divided 

 into types with: (a) submerged leaves, (b) submerged and float- 

 ing leaves, (c) floating leaves, (d) submerged leaves and erect 

 leaves or stems, (e) erect leaves or stems, and lastly (f) marsh 

 plants. 



2. Xcrophytcs. This group includes the plants which have 

 devices for procuring or for storing water, or for limiting trans- 

 piration, adaptations related to dry habitats. They have frequently 

 long tap roots which, in some cases, reach down to a subterranean 

 water supply. There is also often a superficial root system. Xero- 

 phytes of the deserts often have succulent stems, like the cacti of 

 southern and central America. In other deserts, such as the 

 Sahara, succulents are not prominent. A spiny character also 

 characterizes many Xerophytes. 



3. Bog; Xerophytes. These are plants living in the peaty soil 

 of fens and moors, which, though physically wet. are physiologically 

 dry. Such plants can survive a partial or complete drying up of 

 the bog. 



4. Tropopliytes. These are plants with a xerophytic character 

 during the unfavorable season. Thus deciduous trees are xerophytic 

 during the leafless period of winter while other plants survive the 

 unfavorable period by means of their bulbs, rhizomes, or other 

 special structures. 



5. Hygrophytes. These are intermediate between Xerophytes 

 and hydrophytes and are sometimes called Mesophytes. Assimila- 

 tion goes on throughout the whole year, except during periods of 

 frost or when buried by snow. 



6. SciopJiytes. These are plants growing in the shade of 

 forests. They may be hygrophytes or they may be herbaceous 

 tropophytes. 



7. HalopJivtes. These are plants growing in saline soils, and 

 they are characterized by xerophytic adaptations. Many of them 

 are succulent, their leaves and, to some extent, their stems having 

 much water-storing tissue. 



8. Calcicole and Calcifiige Plants. Plants invariably inhabiting 

 calcareous soils are said to be calcicoles, while calcifuge species are 

 rarely or never found in calcareous soil. They are sometimes 

 termed silicic oles. 



