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A TEXTBOOK OF THEORETICAL BOTANY 



and inner walls more than the thickened radial walls. One of two things 

 may then happen. If, as in the majority of flowers, the thickening bands are 

 strongest towards the inner side, then the outer cell walls contract most 

 and the sporangial wall curls outwards, exposing the pollen. If, as in a 

 few cases {Butotmis), the thickening bands are strongest towards the outer 

 side, then the inner cell walls contract most and the sporangial wall curls 

 inwards. This also exposes the pollen, though not so effectively as in the 

 first case. 



The opening process is generally rather slow. A truly explosive opening 

 of the anther, like that of a Fern sporangium, is only known in Ricimis. 

 The so-called explosive dispersal of pollen which occurs, for example, in 

 the Urticaceae, is not due to the dehiscence of the anthers, which is carried 

 out normally, but to the elastic snap of the filaments straightening out 

 when they are released, as we have described above for Pilea (p. 1187). 



Porose dehiscence may imply only a very short stomium. The " pore " 

 is generally longitudinal and then corresponds to a partial longitudinal 

 dehiscence. Some Ericaceae, however, are genuinely porose, in the sense 

 that they have no stomium and the pore is formed by the dissolution of a 

 group of cells. The valvate anthers of Lauraceae, etc., obviously have a 

 U-shaped stomium line and the thickened middle layer of the wall is con- 

 fined to the valve flaps. 



This thickened mechanical layer of the wall has been called the endo- 

 thecium by von Goebel. He points out that although the sporangia of all 

 the Spermatophyta belong to the eusporangiate type, the sporangia of 

 Gymnosperms dehisce by the action of their outer wall layer, or exothe- 

 cium, with the single exception of Ginkgo, while the great majority of 

 Angiosperms dehisce by the action of the inner, endothecial layer. Excep- 

 tions to this rule among the Angiosperms are not common and in almost all 



Fig. 1 172. — Part of a transverse section through the anthc'r of 

 Erica showing the undifferentiated anther wail. (After Go.b:'l.) 



