EXTREME CONDITIONS 449 



The evergreen trees of arid regions, like the evergreen or 

 Hve oaks of the Californian Chaparral and of the Mediter- 

 ranean countries, are even more pronouncedly sclerophyllous 

 than evergreens of temperate countries. Another type of 

 perennial, highly characteristic of semi-arid, open country, 

 is the bulbous geophyte which forms a notable element of 

 the floras of South Africa, Asia Minor, and the Mediter- 

 ranean lands . During the dry season these rest underground 

 as bulbs or corms, and in the winter or early spring, when 

 moist soil is combined with favourable temperature, 

 extremely rapid growth, made possible by the large pre- 

 formed buds and the great store of food, results in a sudden 

 wonderful profusion of flower and foliage. There follows 

 a period of active assimilation, the formation of new buds 

 and food stores, and then the leaves wither and die with 

 the onset of the arid season. We have seen that there is 

 reason to believe that our North European bulbous plants 

 are derived from such types, and that their spring growth 

 has enabled them to live in a forest environment by making 

 use of the only season when the conditions as regards light 

 and the other factors necessary for assimilation are favour- 

 able. The fact that growth of bulbous plants begins early in 

 winter is, of course, familiar to every gardener. 



Frost Resistance. — In temperate climates the unfavour- 

 able season is winter, with frost as the chief danger ; the 

 cell may be directly damaged by freezing, and indirectly 

 the plant may suff"er from temporary excessive transpiration 

 with slow water supply from a frozen soil. The poor light 

 and low temperature in any case reduce assimilation to a 

 minimum, and a covering of snow may stop it entirely for 

 weeks or months. 



Many perennial grasses and herbs, especially rosette 

 plants like the daisy and dandelion, retain their leaves 

 throughout the winter. They form what Lidfors (1907) 

 has called the " winter-green flora." Sheltered by their 

 lowly position, existing in the dampest layer of the atmo- 

 sphere close to the soil, their chief danger lies in direct 

 damage by freezing. Lidfors has shown that in such plants 



2 G 



