io88 A TEXTBOOK OF THEORETICAL BOTANY 



become altered and thickened to form the protective covering or testa of 

 the seed. The matured product of the ovule is therefore a seed, surrounded 

 by a testa and containing an embryo, with or without endosperm. 



Simultaneously the wall of the enclosing ovary becomes altered to form 

 the fruit wall or pericarp, sometimes dry and woody or leathery, some- 

 times fleshy, within which are enclosed the ripe seeds. The other parts of 

 the flower are usually shed during fruit development so that finally the fruit 

 is the only organ left on the pedicel. The fruit may, in its turn, dehisce and 

 release the'seeds, or it may be shed as a whole, with the seed or seeds still 

 enclosed. 



ARRANGEMENTS AND RELATIONSHIPS OF FLORAL PARTS 



The production of flowers is generally regarded as an index of physio- 

 logical maturity and the initiation of flower buds is indeed associated with 

 biochemical changes which mark the close of a developmental phase. The 

 onset of flowering is of course subject to influence by such external factors 

 as temperature and length of exposure to daylight, but these factors must 

 operate upon an organism which is, so to speak, ripe for flowering, in order 

 to produce their efl^ects.* 



Species vary greatly in the time of onset of flowering. The ephemeral 

 weeds, such as Cardamine hirsiita, may produce an inflorescence within a 

 few weeks of germination, but some of the longer-lived forest trees may wait 

 for thirty years before flowering for the first time. The habits of plants in 

 this respect fall into two categories; firstly those which flower but once 

 and then die, which are called monocarpic. They may be either short- 

 lived annuals, or perennials like Agave americana, the giant monocotyle- 

 donous shrub which grows slowly for many years and then throws up its 

 single immense and final inflorescence (Fig. 1057). The same habit obtains 

 among the Bamboos and in several of the Palms. Secondly there are those 

 which flower repeatedly, once or more every year throughout their lifetime, 

 which are classed as polycarpic. This latter class have the great biological 

 advantage that even if the period of immaturity of the individual is prolonged, 

 there will always be a succession of new individuals reaching maturity every 

 season. The monocarpic perennial which flowers only once after a long 

 period of development, proceeds, as it were, by a series of jumps and is 

 plainly in a less favourable and more vulnerable condition than the poly- 

 carpic plant, whose more nearly continuous progress from generation to 

 generation evens out the risks from the climate or from the attacks of enemies. 



Even polycarpic plants do not always flower regularly but are periodic. 

 There are numerous examples, of which we may cite Batiksia (Proteaceae) 

 and the Southern Beech (Nothofagiis). The common Beech (Fagus) also, 

 though it may flower regularly, is only periodically fertile. 



* Flowering maturity is probably connected with the formation of specific growth sub- 

 stances in the plant, since it has been shown that naphthalene acetic acid and other synthetic 

 growth substances can induce flowering in a number of plants. 



