128 MORPHOLOGY OF THE ANGIOSPERMS 



anthers, where the sporangia are isolated in the four corners, there is 

 usually no fusion of sporangia at maturity and the "theca" represents 

 one sporangium (the anther is "tetrathecous"). In some highly spe- 

 cialized anthers, all four sporangia unite to form one chamber, and the 

 "theca" represents four sporangia — CaUitriche, some Cucurbitaceae. 



Anther-dehiscence Types. On the basis of position or direction of 

 dehiscence, stamen types — better called dehiscence or anther types — 

 are distinguished in taxonomic use as introrse, when pollen is freed 

 from the anther directly or obliquely inward in the Hower (adaxially); 

 eoArorse, when dii-ectly or obliquely outward (abaxially). (In some 

 genera, anthers that appear introrse in the bud appear extrorse in the 

 flower — and those extrorse in the bud, introse in the Hower — because of 

 changes in form or structure of the filament or of movement of the 

 anther on the filament apex.) When pollen is shed laterally, dehiscence 

 is termed Introrse; in older usage, "normal." Introrse anthers character- 

 ize the majority of angiosperms; extrorse anthers characterize a rather 

 small group of families — Calycanthaceae, Myricaceae, Cucurbitaceae, 

 Fumariaceae, Lardizabalaceae, Aristolochiaceae, Iridaceae, Juncagina- 

 ceae, Potamogetonaceae, and a few others. Some families — Liliaceae, 

 Alismataceae — have both types; and, in the Magnoliaceae, Liriodendron 

 has extrorse anthers, the other genera, introrse; in Fagopyrum and 

 Persea, one whorl of stamens is extrorse, the other introrse; in Cinna- 

 momum, two whorls are introrse, one extrorse; in CommeJina, two of 

 the three stamens are extrorse, the third, introrse. Anthers introrse in 

 the bud may become extrorse at flowering — Geraniaceae, Caryophyllaceae, 

 some palms. The latrorse position is intermediate between the primi- 

 tive laminar and the advanced extrorse and introrse — Ranunculaceae, 

 Butomaceae, Menispermaceae. Figure 52 shows a series of theoretical 

 steps in the modification of the ancestral lamina of the microsporophyll 

 to form the latrorse and, ultimately, extrorse or introrse anther. Even in 

 anthers that, in form, are extrorse or introrse, discharge of pollen may 

 be in a lateral direction — Solarium, Lonicera, Valeriana, Primida. Such 

 dehiscence as this characterizes genera but not larger taxa. All anther 

 types could have been derived from sporophylls with, primitively, 

 either adaxial or abaxial sporangia. Orientation of sporangia as indicat- 

 ing extrorse or introrse is commonly apparent early in the ontogeny of 

 the stamen, but often, in early stages, the sporangia are symmetrically 

 placed near the four angles of the primordium and the final position 

 is determined by differential growth. This growth may be sti'ong and 

 even suggest cambiumlike activity (Fig. 57C, D). 



The terms applied to dehiscence have much descriptive and taxono- 

 mic value and are morphologically significant, in that an evolutionary 

 sequence is obvious from the simple extrorse and introrse dehiscence 



