i 3 o ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



plants their characteristic habit, by inducing short, curved, and crooked 

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

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

 possessed of long internodes. In Mediterranean and other subtropical 

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

 medium height, but in moist valleys they vary from this form 

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

 leaves are often closely packed, the ramification being extraordinarily 

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

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

 fragrantissima, Artemisia Herba-alba, and Cleome arabica, 1 all in the 

 North African desert ; the globular bushes of Astragalus and Genista 2 

 in Corsica ; Draba alpina, 3 Silene acaulis, 4 species of Saxifraga, and 

 many mosses of cushion-growth in arctic countries 5 ; Androsace helvetica 

 and others in the Alps. 6 The high mountains of South America and of 

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

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

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

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

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

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

 is Raoulia mammillaria living in New Zealand. 7 



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

 another factor. Dense ramification and tufted growth confer a benefit 

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

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

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



ROSETTE-PLANTS 



Many xerophytes have their leaves arranged in rosettes on shoots 

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

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

 and deserts, among epiphytes and tropical lithophytes. 8 The brevity 

 of the internodes and the consequent arrangement of the leaves cannot 

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

 identical. In many Bromeliaceae the rosette serves to collect and retain 

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

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

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

 because the leaves spreading over the soil are not so much exposed to 

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

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

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

 Meigen 9 also remarks that the leaves of many rosette-plants by over- 

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

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



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 Hesselman, 1900. Schroter, 1904-8. 



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



