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130 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect. h.\p 



plants their characteristic habit, by inducing short, curved, and crool f^' 

 shoots and stems, with short internodes, and with a feeble or irregi 

 production of buds : abundant moisture causes shoots to be long i 

 possessed of long internodes. In Mediterranean and other subtropi 

 countries with winter-rain, many species assume the form of shrubs I ^S^ 

 medium height, but in moist valleys they vary from this fo |iWO 

 up to that of a tall tree. In scrub or in the desert, the branches a 

 leaves are often closely packed, the ramification being extraordinar 

 dense, and the plant as a whole being compact and rounded in fo] 

 (hemispherical, or cushion-like) ; as examples may be cited Achil' 

 fragrantissima, Artemisia Herba-alba, and Cleome arabica,^ all in t 

 North African desert ; the globular bushes of Astragalus and Genist 

 in Corsica ; Draba alpina,^ Silene acaulis,* species of Saxifraga, ai 

 many mosses of cushion-growth in arctic countries ^ ; Androsace helveti( 

 and others in the Alps. The high mountains of South America and 

 all other lands display many examples of cushion-like shrubs or herl 

 which appear as if cleanly bitten or clipped ; for they are rounded 

 dense in growth or even solid, and have their numerous shoots, leave 

 and remnants of these closely packed together : as examples may , 

 cited the umbelliferous Azorella and Laretia, species of Oxalis, ar 

 Cactaceae in South America. One of the most remarkable cushion-plan' 

 is Raoulia mammillaria living in New Zealand.'^ 



Everywhere the cause is the same dryness, occasioned by one c 

 another factor. Dense ramification and tufted growth confer a benef 

 upon the plants, in that their young shoots are thereby better shielde 

 from transpiration ; they protect each other and are in turn protecte 

 by older shoots from the desiccating action of wind in arctic countries. 



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ROSETTE-PLANTS 



Many xerophytes have their leaves arranged in rosettes on shoot 

 which resemble the first year's growth of biennial dicotyledons : rosettt 

 plants are encountered in arctic countries, at alpine altitudes, on steppe 

 and deserts, among epiphytes and tropical lithophytes.^ The brevit 

 of the internodes and the consequent arrangement of the leaves canno en 

 perhaps always be explained in the same manner, nor is the utility alway 

 identical. In many Bromeliaceae the rosette serves to collect and retai: 

 water ; in other plants, such as Agave, it may be that the leaves formin, 

 the rosette are better screened from the sun and from excessive transpira 

 tion. In arctic and alpine plants the low rosette-shoot may benefi 

 because the leaves spreading over the soil are not so much exposed t< 

 desiccating winds, also because these leaves are situated in warmer ai 

 and are better able to obtain heat from the soil. It is probable that ii 

 the desert they can utilize to good advantage the dew deposited by night 

 Meigen ^ also remarks that the leaves of many rosette-plants by over 

 lapping one another produce niches screened from the wind, and thu 

 reduce their transpiration. Rosette-plants thrive among open and lov 



' Volkens, 1887. * Massart, 1898 ; Rikli, 1903. 



' Figured in Kjellman, 1884, p. 474. 



* Figured in The Botany of the Faroes (Copenhagen, 1901-8), p. 993. 



* Andersson and Hesselnaan, 1900. * Schroter, 1904-8. 

 ' See Section IX. * See p. 27. ' Meigen, 1894. 



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