,i84 A TEXTBOOK OF THEORETICAL BOTANY 



the halving of the anther by chorisis {Adoxa) or by the elongation of the 

 connective (Labiatae; see p. 1189). The monosporangiate anthers of Poly- 

 gala may also be attributed to chorisis of the anther alone, the filaments 

 being undivided and indeed concrescent. An exceptional increase in the 

 number of sporangia most frequently arises through transverse septation 

 (some Annonaceae, Lauraceae and Onagraceae) or in other cases by septa- 

 tion in several directions, isolating numerous sporangial sacs, which may be 

 irregularly distributed {Viscum, Rhtzophora, and Mimosaceae) (Fig. 1156). 

 This multisporangiate condition is probably a derived condition rather 

 than a primitive one, if we have regard to the systematic positions of the 

 plants which display it. 



Fig. 1 1 56. — Calliandia tetragona (Mimosaceae). A, 

 Stamen after dehiscence with four pollinia in each half 

 anther, each polhnium in a separate loculus. B, Un- 

 opened anther. C, I^^arHer stage before anther be- 

 comes vertical. D, Polhnium with sticky pad at top 

 which attaches it to visiting insects. {After Goebel.) 



When an anther is mature the outer walls of the pollen-sacs split open, 

 exposing the ripe pollen, which is then removed by the wind or by insects, 

 etc. This is called dehiscence. If this occurs on the inner (adaxial) side 

 of the anther it is said to be introrse, which is the commonest condition; 

 if it occurs on the abaxial side, which is comparatively infrequent, it is 

 called extrorse. The latter condition usually implies that the sporangia 

 themselves are directed abaxially either permanently or temporarily. Thus 

 in some species of Gentiana the anther is at first adaxial but it turns over 

 vertically into the abaxial position before it dehisces. Some flowers, e.g., 



