FORCING: REPRODUCTION 457 



ment, and that the cause of such rhythm must be sought 

 in periodic changes inherent in the complex colloids of the 

 plasma. 



§ II. The Reproductive Phase , 



The formation of reproductive organs takes place after 

 a period of vegetative growth which in normal conditions 

 is specific. In the ephemeral or annual plant reproduction 

 closes the life of the individual after a few weeks or 

 months ; in the biennial in the second year. In perennials 

 the conditions are very varied ; in many herbaceous species 

 flowering begins early, after one or several vegetative seasons, 

 and recurs annually, as in the primrose and wood anemone. 

 In bulbous plants the bulb raised from seed may not flower 

 for several years. In shrubs flowering may begin quite 

 early ; in trees it is often long delayed. The beech first 

 flowers at an age of 40 to 50 years, the oak at 40 years, the 

 hazel at 10 years, the field maple at 25 years, the elm at 

 40 years, the fir at 60 years, the pine at 15 years. In most 

 trees the flowers are laid down in the bud nearly a year before 

 they open, and the same is true of bulbous plants. The age 

 of first flowering varies in different climates and conditions ; 

 it is always later in trees grown in close canopy. While 

 some trees blossom and fruit every year, in many abundant 

 seed is produced only in periodically recurring " mast " 

 years. The mast years of a beech occur at intervals of 

 10 to 15 years, while for the pine and the hazel the intervals 

 are 3 to 5 and 2 to 3 years respectively. The period is 

 evidently determined partly by external factors and partly 

 by internal, for different individuals growing together may 

 flourish in different years, and the periods are variable. 



In some perennials flowers are produced only once. 

 The famous, but miscalled, century plant, Agave americana, 

 grows vegetatively for 8 to 10 years and then produces one 

 immense inflorescence and dies ; in the less favourable 

 climate of Europe its life may be prolonged to several decades 

 before flowering takes place. An even more striking 



