942 ECOLOGY 



roots frequently are horizontal or even ascending rather than descend- 

 ing. Rhizomes are greatly developed, accounting in large part for the 

 rapid invasion of ponds by swamp plants. A somewhat remarkable 

 feature is the abundant development of vertical chlorophyll-bearing 

 organs, whether leaves (as in the flags) or stems (as in the rushes). 

 Among the most plastic of plants as to leaf form and structure are the 

 amphibious plants; in view of the rapid transformation of ponds into 

 swamps, such plasticity permits certain species to dominate in two dis- 

 tinct successional stages. 



Bogs or moors. The mature vegetation of a peat bog contrasts most 

 strikingly with that of an ordinary swamp, although the early stages may 

 be quite the same in both cases. While a number of plants are common 

 to swamps and bogs, there are many kinds of plants which are more or less 

 peculiar to bogs, the most noteworthy being those with such xerophytic 

 features as prominent palisade tissues and cutin, dwarfness of habit, 

 and high osmotic pressure. Among the bog xerophytes are many ericads 

 (such as the cranberry, leather leaf, and Labrador tea) and conifers; 

 that some of the bog plants are true xerophytes is shown by the fact that 

 a number of species are common to bogs and to dry rocky cliffs. The 

 peat moss (Sphagnum) is especially characteristic of bogs, as are many 

 orchids, and it is in bogs that most carnivorous plants are found. As 

 compared with mesophytic habitats or with ordinary swamps, bogs 

 present conditions that are deleterious for the majority of plants ; indeed, 

 some of the plants which are characteristic of bogs (notably the tamarack) 

 thrive much better elsewhere, suggesting that they " tolerate " bogs 

 rather than " select " them. An analysis of the bog problem is beyond 

 the scope of this book, but some points that bear on the matter have been 

 suggested elsewhere (p. 537). 



Maritime associations. Plants that grow in salt water or in salty soil 

 have been denominated halophytes. The submersed halophytes are 

 chiefly algae, which sometimes reach gigantic size, and which differ in 

 color, being green, red, or brown. Most of the larger algae are attached 

 to rocks by anchoring organs, namely, the holdfasts or rhizoids; some 

 rise and fall with the tide, bladders filled with air often facilitating their 

 buoyancy. Salt marshes show stages in succession comparable to those 

 of ponds, but the species involved are very different. Emersed halophytes 

 are strikingly xerophytic in their characteristic features, palisade tissue 

 being prominently developed, and often the epidermis is highly cutinized. 

 The most striking feature of salt marsh halophytes, taken as a class, 



