iio6 A TEXTBOOK OF THEORETICAL BOTANY 



main united in their basal regions. Chorisis is most frequently observed in 

 stamens and we shall give further examples later in this chapter under the 

 heading of " The Androecium ". In double flowers however, both stamens 

 and petals may be affected by it, the former being, in addition, modified into 

 petaloid structures. Where the division takes place along the radial plane, 

 we speak of lateral chorisis and where it is tangential to the flower it is called 

 parallel or serial chorisis. Occasionally both may occur together. Chorisis 

 is not to be regarded simply as a monstrosity, though it may sometimes 

 produce abnormal forms Hke double flowers. On the contrary, it is often a 

 constant or even a distinguishing feature of whole genera like Butotnus and 

 Scleranthus (Fig. 1078), or some genera in the Cruciferae (see p. 1176). 



A B 



Fig. 1078. — Floral diagrams. A, Biitomiis. B, Sclerantliiis. 



We have referred above to the fact that in the comparatively un- 

 specialized spiral flowers the numbers of parts are usually large and indefi- 

 nite. It is an observed fact that parts of any one kind which are numerous 

 have smaller rudiments and less constant numbers than parts of which there 

 are only a few. This rule is one with a widespread application and it can be 

 observed in animals as well as in plants. Among Angiosperms, the cyclic 

 arrangement necessarily Hmits the numbers of organs, unless, like the 

 stamens of the Poppy, they are very small, and flowers of specialized 

 structure are aU cyclic and oligomerous, that is, have small numbers of parts. 

 Floral evolution has, in fact, been accompanied by much material reduction, 

 although it is associated with greater sexual efficiency. 



When we compare extremes, such as Nymphaea, with very many parts, 

 and Galium, with few, the general evolutionary tendency towards oligomery 

 is undeniable, but in the intermediate regions fluctuations of the numbers 

 of parts, or meristic variation, are quite frequent, involving both meio- 

 mery or reduction, and pleiomery or multiplication. Only the most 

 advanced families show a high degree of fixity of numbers. 



Meiomery may involve abortion, that is the disappearance of parts, 

 sometimes partially, with a rudiment remaining, sometimes so completely 

 that no trace remains, even in the microscopic anatomy. 



Meiomery is more frequently due to fusion of members than to sup- 



