DEHISCENCE OF THE ANTHER 165 



The pollen of the monocotyledons appears to be, in general, more 

 primitive than that of the dicotyledons; the monocolpate type is dom- 

 inant, and the tricolpate type not present. It has been suggested that the 

 prominence of the monocolpate type is perhaps connected with phylo- 

 genetic homogeneity, hygrophily, and geophily. 



The characteristic monocolpate pollen of the monocotyledons has been 

 called "evidence of their antiquity," because a derivation from the tri- 

 colpate Ranunculaceae — as often suggested for the monocotyledons — 

 cannot be considered. The primitive monocolpate pollen of the mono- 

 cotvledons is evidence at least of their very early origin, whatever their 

 ancestral stock may have been, 



DEHISCENCE OF THE ANTHER 



Pollen is commonly shed from the anther tlirough longitudinal, slitlike 

 openings in the anther or lamina wall — longitudinal dehiscence. Trans- 

 verse slits occur occasionally — Alchemilla, Hibiscus, Euphorbia, Chrijso- 

 splenium. Where the sporangia are united in pairs, the longitudinal 

 opening follows the furrow between the sporangia of a pair along the 

 line of sporangial fusion. Dehiscence by small, rounded openings, 

 "pores" — poricidal or porosc dehiscence — is characteristic of a few 

 families — Ericaceae, Epacridaceae, Tremandraceae, Melastomaceae 

 (most genera), Myrsinaceae, some Leguminosae, Ochnaceae, Solanaceae. 

 Pores are usually located at the distal ends of the sacs, rarely at the 

 proximal ends. In the anthers of some Ericaceae, the distal ends are, 

 morphologically, the proximal ends; the anther is inverted in ontogeny. 



Arrangement of the sporangia in pairs brings about variety in longi- 

 tudinal dehiscence. In laminar stamens, the sunken sporangia often de- 

 hisce independently; the sporangia have individual endothecial caps. In 

 anthers where the sporangia lie close together in lateral pairs, dehiscence 

 is usually by a common slit in the furrow between the two sporangia. 

 As the pollen matures, the anther-sac wall separating the two sporangia 

 breaks down, and the pollen grains mingle as they escape into the 

 furrow (Fig. 67F). Where the four sporangia are isolated in the corners 

 of an angular anther, dehiscence is from each sporangium separately and 

 in a more or less lateral direction. 



Dehiscence results generally from hygroscopic shrinkage of the fibrous 

 layer. It may be initiated in regions of existing structural weakness, 

 such as absence of epidermis along the line of dehiscence. Where de- 

 hiscence is longitudinal, continued drying brings about retraction of the 

 borders of the openings, and the chambers are opened widely. Changes 

 in atmospheric humidity may cause repeated opening and closing of the 

 anther sacs. The pollen may all be freed at once or may gradually 



