734 ECOLOGY 



species which develop into trees on intermediate slopes, where the rainfall is greater. 

 These and other dwarfed forms of arid soils and climates probably are due to the 

 excessive transpiration in proportion to the limited absorption. 



Dwarfing in bogs. When the bald cypress (Taxodium) is grown in upland 

 parks, the trunk is excurrent and the branches symmetrical, but in its natural swamp 

 habitat the main shoot dies after a number of years, and the subsequent vigorous 

 development of lateral shoots results in a spreading crown. Possibly the imperfect 

 absorption which is characteristic of swamps makes it impossible for the water column 

 to rise as high as in other habitats. In western bogs, Pinus contorta, elsewhere a 

 slender tree, frequently becomes a gnarled and sprawling shrub, doubtless because 

 of the unfavorable conditions for absorption. 



Nanism. Among the mutants of Oenothera Lamarckiana (p. 288) 

 there appeared some forms that were much smaller than the parents 

 or the other mutants, and as these dwarfs reproduce true from seed, they 

 were given the specific name, Oenothera nanella. Plants of this sort, 

 whose dwarfness appears to be inherent, rather than caused by external 

 conditions, are said to exhibit nanism. Similarly, species whose in- 

 dividuals are inherently large are said to exhibit gigantism. Experi- 

 mental cultures have shown that of the many dwarf xerophytes of eastern 

 Sweden, some are inherently dwarf (obligate dwarfs) , illustrating nanism, 

 while others (facultative dwarfs) develop readily into tall plants when 

 grown in favorable conditions. In obligate dwarfs all of the organs 

 commonly are reduced, but in facultative dwarfs the roots and often 

 the flowers are as large as in full-sized individuals; however, experimen- 

 tation is necessary in order to determine adequately whether any given 

 dwarf is facultative or obligate. 



Asymmetric stems. Most aerial stems tend toward symmetry in 

 their branch development, but asymmetry often is seen, as in trees at 

 the edge of a forest, which branch profusely toward the open, while 

 they are nearly branchless toward the forest interior, probably because 

 of insufficient light. The most striking cases of one-sided trees, how- 

 ever, occur along seacoasts, branch development often being almost 

 entirely inhibited on the seaward side, so that most of the branches 

 point landward. Usually the crown of the tree presents an even slope 

 upward from the seaward side, giving a peculiarly wind-swept aspect, 

 and indicating that the inhibition of branch development is diminished 

 landward (fig. 1056). Two theories have been put forward to account 

 for such asymmetry, neither having adequate experimental evidence; 

 the one maintains that the stronger transpiration on the seaward or 

 windward side causes the branches to die earliest there, while the other 



