448 THE BIOLOGY OF FLOWERING PLANTS 



far greater number than of perennials ; 1 22 spring up in the 

 winter, and 44 in the summer rainy season. It is as seeds 

 that the plants pass through the dry seasons. 



In climates such as ours, where the winter conditions 

 are the least favourable, because of poor light and low 

 temperature, annuals are also numerous, though they form 

 a minority of the flora, Most of our common annuals, 

 however, are weeds of cultivated land, e.g. several of the 

 speedwells, the hemp-nettles, the goosefoot, the fumi- 

 tories, the groundsel, the charlock, the shepherd's purse. 

 These are not really native plants. They have followed the 

 plough, and travelled with man from a centre of distribution 

 which probably lay in Eastern Europe, Asia Minor, and the 

 Mediterranean basin, where semi-arid summer conditions 

 exist. To these they are primarily related, though their 

 periods of vegetation have altered to suit the conditions of 

 their new homes. Frequently their seeds germinate in 

 autumn so that they pass the winter in a state of slow 

 growth. Most can produce several generations in a year. 

 It is of interest to compare the native species of Veronica 

 of the section Chamaedrys, typically woodland and marsh 

 plants, and all perennial, with the weed species of the 

 section Omphalospora, growing in waste places, and annuals. 

 Such annuals as Vicia lathyroides, Myosotis collina, Erophila 

 vema, Teesdalia nudicaults, and Aira praecox may be true 

 natives. They grow in dry exposed situations, on sand 

 dunes, on the turfy tops of stone walls, and flower in spring, 

 passing the dry summer months as dormant seeds. Even 

 in temperate climates the annual habit may be related to 

 drought rather than to winter frosts. 



Arid Conditions. — Plants which live for several years 

 may exist in the nearly uniform conditions of some regions 

 of the tropics, and vegetate continuously, or they may be 

 exposed to a periodic incidence of heat and drought or of 

 cold. We have already seen that several types of tropical 

 and sub-tropical forest and scrub, the savannah forest, the 

 thorn woodland, and the thorn scrub, are characterised by 

 trees and shrubs which cast their leaves in the dry period. 



