THE ANGIOSPERMAE 



1151 



the plant is ready or ripe for flower formation and it may be that the mecha- 

 nism is put in motion or directed by the action of certain material agents, 

 which we may think of as " flower-realizers " in the sense that they induce 

 flower formation in a system already prepared for it. Readiness for anthesis 

 is a definite physiological condition, localized in certain parts of the plant, 

 as shown by the fact that plants grown from cuttings of flowering shoots, 

 though slow to establish their own roots, give plants which grow slowly 

 and flower freely, but cuttings of vigorous, vegetative shoots form roots 

 quickly and give plants with luxuriant vegetative growth and few flowers. 

 The underlying physiological factors in this disposition to flower are still 

 little understood, but it is known that the relative lengths of exposure to 

 light and darkness are important, and may be decisive in controlling flower- 

 ing, as we shall see in our treatment of physiology in a later volume. 



Fig. 1 1 27. — Cercis siliqiiastrum. Fascicles of flowers on an 

 older branch. An example of nudiflory and also of 

 ramiflory, one of the forms of cauliflory. 



Flowering usually precedes or follows, rather than accompanies, the period 

 of greatest vigour in growth. Some plants indeed flower before the leaves 

 appear, when there is no other growth in progress. Others flower, like 

 Colchicum, after the leaves have died off, either condition being spoken of as 

 nudiflory. Many species flower on the extending shoots of the current 

 season, but far more flower on older shoots, of the previous or still earlier 

 seasons. Tropical woody plants have the peculiarity, in many cases, of 

 flowering on the oldest branches or even on the main trunk, which is called 

 cauliflory (Fig. 11 27). See also p. 1074. 



The expansion of the flower is brought about by a change in the growth 

 relationships of the perianth members. In the bud stage the close associa- 

 tion of parts is due to hyponastic growth, that is growth which is greater 

 on the abaxial than on the adaxial side. This gradually diminishes to zero 

 and is replaced by epinastic growth, or excess growth on the upper surface, 



