78 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



at all, even though vegetative development is good. Again, the 

 English Daisy [Bellis perennis) dies when grown continuously in a 

 warm greenhouse : with a day-temperature of 26"^ C. the plants 

 survive only at night temperatures below 10° C, and flower 

 abundantly only at still lower night temperatures. Accordingly 

 such plants are unable to reproduce normally in consistently warm 

 climates. In other instances, closely related strains may differ 

 markedly in their night-temperature requirements for flowering ; 

 these requirements may be decisive in determining which strains, 

 if any, can flourish in a particular area. This is true of Tomatoes, 

 where fruit-set is dependent upon a very narrow range of tempera- 

 tures — a phenomenon which is reflected in very large differences in 

 yield in varying circumstances and with different strains having 

 even slight deviations in optimal requirements. Numerous instances 

 are now known in which, for these or other reasons, slight differences 

 in the temperature-response of plants will exert a controlling influence 

 on their local survival and consequently on their distribution. 



In the many parts of the world that have markedly varying seasons, 

 one of the main concerns of their plants is to tide over unfavourable 

 periods — usually of cold or drought. To this end is expended a 

 good deal of what might be called evolutionary ingenuity, and also 

 much physiological effort — for example, in storing food for the 

 adverse period and for subsequent development. Among the most 

 successful methods employed is the annual habit, in which the 

 adverse period is evaded by being passed over in the form of a 

 resistant seed or fruit, the parent having meanwhile died. Numerous 

 common weeds, such as Chickweeds {Stellaria spp.) and Shepherd's- 

 purse {Capsella bursa-pastoris), practise this method, as do many of 

 the diminutive ' ephemerals ' which blossom so pleasingly after rain 

 in the less extreme deserts. In the Arctic and some other rigorous 

 regions, however, the growing-season, though fairly regular, is too 

 short and cool to allow full development — from seed through seedling 

 and adult to flower and seed again — in a single season. Accordingly 

 almost all the plants there are perennial, passing the adverse winter 

 period in a more or less resistant and dormant state — often after 

 dying down (in the case of herbs) or losing their leaves (in the case 

 of deciduous shrubs and trees). Most Mosses and some other plants 

 have the fortunate capacity to endure drought by drying up almost 

 entirely without ill effect, resuming normal life again when moistened. 

 All these, as well as any growth-responses they involve, are physio- 

 logical activities (or inactivities) and, in a sense, adaptations to 



