The Measurement of Flowering • 7 



differ somehow from typical perennials that flower and fruit over 

 tens or even hundreds of years without evincing any ill effects. 



Many studies of flower initiation and development under nat- 

 ural temperate-zone conditions have been made on individual 

 species. A survey of a large number of species in Britain was re- 

 ported by Grainger (1939). By determining the times of flower 

 initiation, bud development, and subsequent anthesis (flower open- 

 ing), Grainger distinguished three classes of temperate-zone plants. 

 Direct-flowering plants are those in which development through 

 anthesis follows on initiation without interruption; this is perhaps 

 the commonest type of flowering behavior, found in both mono- 

 carpic and perennial plants. Initiation and anthesis may occur 

 either together with the maximum vegetative growth, as for ex- 

 ample in bluebells (Campanula) and mint (Mentha), or at the 

 period of minimum vegetative growth (winter or early spring) as 

 in Saxifraga. A second class, indirect-flowering plants, contains 

 those species in which a distinct period of rest intervenes at some 

 stage between initiation and anthesis. Here again, initiation may 

 coincide with the period of maximum vegetative growth, as in 

 many fruit trees (Pyrus, Prunus) and in Anemone, or with the 

 period of minimum vegetative growth, as in many bulb flowers 

 (Tulipa, Narcissus) that initiate flower primordia in summer after 

 the leaves wither. A third class, cumulative-flowering plants, form 

 primordia over a long period of time, in regular succession, but 

 anthesis of all occurs in a brief period. A number of weed species, 

 notably dandelion (Taraxacum), are in this class. Grainger distin- 

 guished a fourth class, climax-flowering plants, not found in the 

 temperate zone but including long-lived monocarpic plants such as 

 the bamboos mentioned above. 



Most experimental studies of flowering have been conducted 

 on plants of Grainger's first class— direct-flowering plants initiating 

 in the period of maximum vegetative growth. Other types have 

 been studied, however, as will appear in the succeeding chapters. 

 Unfortunately, but for obvious reasons, there has been little if any 

 experimentation on long-lived monocarpic plants. 



THE MEASUREMENT OF FLOWERING 



The general structure of experiments on flowering is obvious- 

 groups of plants given various treatments are kept under observa- 



