6 • Background 



to be ripe-to-flower. This concept will be considered in connection 

 with work requiring it, notably in Chapter Seven, but by itself it 

 explains little about the physiological events taking place and 

 seems not to reflect any basic morphological conditions common to 

 all plants. 



For more detailed treatments of the topics discussed here, see 

 Lawrence (1951), Esau (1953), and Foster and Gifford (1959). 



NATURAL HISTORY OF FLOWERING 



Most of what is known about flowering is based on work done 

 either with plants native to the temperate zone or with cultivated 

 plants. Flowering times and habits particularly have been studied 

 more thoroughly in the higher latitudes than in the tropics. This 

 limitation should be kept in mind in any discussion of flowering 

 habits and physiological mechanisms. The general state of igno- 

 rance on flowering in the tropics, and particularly its seasonal 

 aspects, is well summarized by Richards (1957, pp. 199-204). 



Plants are often classified as annual, biennial, or perennial. 

 Under these familiar terms a plant either germinates, flowers, and 

 dies within a single season, germinates and develops during one 

 season and flowers and dies in the next, or persists for many years 

 flowering repeatedly. Such classifications are not always physiolog- 

 ically meaningful, although, as will appear later, many biennials 

 can be regarded as annuals in which a low-temperature treatment 

 is required for flower initiation. But many plants commonly called 

 annuals do not die after flowering and fruiting in all climates; 

 they may be tropical perennials able to survive or cultivated as 

 annuals in cooler regions. 



There might be more meaning, both ecological and physiolog- 

 ical, to a classification into two groups— the first being perennials, 

 defined as above, and the second, a group called monocarpic plants. 

 Under this term can be classified true annuals, such as the edible 

 pea (Pisum sativum), biennials, and certain others, all having in 

 common the behavior of flowering only once, with fruiting fol- 

 lowed by death. This group then would include plants such as the 

 century plant (Agave) that may develop from five to twenty or 

 more years before flowering, and many tropical bamboos, with life 

 spans from two to perhaps over fifty years. Such plants clearly 



