STEMS 



733 



as elsewhere, the destruction of the terminal shoot is followed by the 

 development of many lateral branches, whose subsequent destruction 

 results in a still greater number of new laterals, and so on, until there is 

 at last an inextricable tangle of branches. The tortuous descending 

 branches so characteristic of alpine conifers probably are associated 

 with severe mechanical factors, such as strong winds, the weight of the 

 winter snow, and snowslides. Even below the " timber line," tree 

 trunks sometimes bend down-hill at the base, owing to the weight of 

 snow borne by the plant when it was a 

 young and flexible sapling. 



Arctic dwarfs. The polar regions, like the 

 mountain tops, are characterized by dwarf vege- 

 tation, composed largely of cushion herbs, 

 rosette herbs, mat-forming plants, and Krumm- 

 holz. The Krummholz is made up largely of 

 dwarfed specimens of woody plants (e.g. larch, 

 spruce, and birch) which in more genial climates 

 develop into trees. While not experimentally 

 attested, it is likely that the factors involved are 

 similar to those that are supposed to obtain 

 on mountains; transpiration, perhaps, is less 

 than in alpine regions, on account of the less 

 intense light and the greater atmospheric pres- 

 sure, but absorption also is less, because the 

 soil is more cbnstantly cold. 



Dwarfing in arid situations. Xerophytic 

 vegetation, such as that of dry rocks and sand, 

 often is dwarf, though much less so than that of 

 alpine and arctic regions, and the dwarfness is 

 less clearly due to the surrounding conditions. 

 The stem of Equisetum hyemale, which com- 

 monly is unbranched in mesophytic and swampy 

 habitats, often is much branched in dry, exposed 

 situations, the destruction of the terminal shoot 

 by excessive transpiration or otherwise being 

 followed by a strong development of lateral 

 branches, as in the Krummholz (figs. 1054, 

 1055). The mesquit (Prosopis juliflora), which 

 is a tree in the moist river bottoms of the arid 

 Southwest, becomes a sprawling shrub in dry, 

 exposed soil (see also fig. 725). The lower slopes 

 of a mountain in a prairie or desert region often 

 have Krummholz which is quite comparable to 

 that toward its summit, and it is made up of 



FIGS. 1054, 1055. Stem varia- 

 tion in the scouring rush (Equisetum 

 hyemale') : 1054, an erect, un- 

 branched stem, terminated by a 

 strobilus (s), being the form com- 

 monly seen in protected situations ; 

 1055, a much-branched stem from 

 an exposed sand dune; following 

 an injury to the terminal shoot (m) 

 there has been conspicuous regener- 

 ation, a number of latent buds hav- 

 ing given rise to lateral branches (/) ; 

 note the absence of foliage leaves; 

 c, scale leaves. 



