1178 A TEXTBOOK OF THEORETICAL BOTANY 



Each flower has apparently ten stamens, but each bears only a half-anther 

 and the ontogeny (Fig. 1151) shows plainly the radial splitting of the five 

 primordia, each half giving rise to a separate structure. The very numerous 

 stamens of Papazer, occurring in a flower of dimerous symmetry, are also 

 due to repeated chorisis. 



Chorisis occurs abundantly in the formation of some double flowers, 

 where the extra petals may be derived, as in the Carnation, from modified 

 stamens which have been multiplied by both radial and tangential chorisis. . 



The multiplication of stamens by chorisis is exceptional but on the other 

 hand, reduction of the androecium appears to have been a general feature 

 of floral evolution, especially in the Dicotyledons. Indefinitely large 

 numbers of stamens are characteristic of only a few families with simple, 

 actinomorphic flowers, while in the great majority of families the number is 

 fixed and bears a close and constant relation to the number of perianth 

 segments. Even so we can see a further tendency to reduction from 

 pentacyclic flowers, with two whorls of stamens, to tetracyclic flowers which 

 have only one whorl, the latter condition being predominant in the most 

 advanced families. Flowers with three whorls of alternating stamens are 

 rare. Three whorls are present in some of the Rosaceae, including Rosa, 

 another example being the male flower of Laurus nubilis. The innermost 

 whorl is, in this case, superposed on the outer whorl and its anthers are 

 reversed so as to face the outer stamens. 



Although the number of stamens in a whorl is generally related to the 

 perianth numbers, there are some cases in which the number in the inner 

 whorl is related to the number of carpels rather than to the petals. An 

 example is the case of polygonaceous flowers with a pentamerous perianth 

 and eight stamens, of which five form the outer whorl and three form the 

 inner whorl, corresponding to the three (united) carpels. Here the small 

 size of the receptacle seems to have induced reduction in the whole central 

 region of the flower. 



The biological value of reduction in the androecium, which is so constant 

 a feature in all the advanced families, is easily understood when we recollect 

 that pollen is an expensive product from the point of view of metabolism. 

 Not only are the pollen grains non-vacuolate and therefore contain an 

 unusually large amount of protoplasm, but they are also well stocked with 

 reserve food-materials. Many flowers with numerous stamens are devoid 

 of nectar and are only visited by bees to collect pollen, which, to the plant, 

 is an extravagant way of ensuring cross-pollination. The contrast between 

 the two types of flower is shown strikingly in a comparison between the 

 related genera Papaver and Fumaria. The former is actinomorphic, has no 

 nectar and has a large and indefinite number of stamens; while the latter is 

 zygomorphic, produces nectar and has only two stamen fascicles, totalling 

 four complete anthers between them. 



Suppression of stamens is a widespread phenomenon and examples 

 can be seen in most families. The suppressed stamens may be represented 

 by vestigial organs, termed staminodes, which may have other functions, 



