392 MORPHOLOGY OF THE ANGIOSPERMS 



Annonaceae 



The Annonaceae, a large, chiefly tropical family of about eighty 

 genera of trees, shrubs, and vines resemble the Magnoliaceae in both 

 vegetative and reproductive characters, especially in general flower 

 structure. The flowers are large, bisexual (rarely unisexual); the peri- 

 anth is whorled, of three series, the inner forming a "biseriate" corolla; 

 the sporophylls are numerous and spirally arranged, but the carpels in 

 some genera are reduced to few and definite in number. As in the 

 Magnoliaceae, there are no staminodes. The gynoecium is apocarpous 

 but becomes syncarpous in fruit in some genera ( Fig. 139 ) . The stamens 

 are short, with thick connective, much enlarged at the tip, and the 

 sporangia are dorsal or dorsolateral. The ovules range from numerous 

 to one or two, and the endosperm is ruminate. 



The family differs from the Magnoliaceae chiefly in its reduced, 

 whorled perianth (in this, resembling Liriodcndron and the more ad- 

 vanced species of Magnolia), and in its estipulate leaves, dorsal micro- 

 sporangia, and ruminate endosperm. 



The inflorescences of various genera in this family give evidence that 

 the solitary flower has been derived by reduction from a determinate 

 cluster (Fig. 40). Bracts on the peduncle, in some genera, seem to show 

 position of lost flowers. The corolla, appearing as two whorls of three 

 petals each, is shown by anatomy to represent one whorl; the traces of 

 all the petals arise at the same level on the receptacle. In some genera, 

 three of the six petals are vestigial, apparently a stage in the formation 

 of a trimerous — one whorl — corolla. The thick, broad connective of tlie 

 stamens may partly hide the sporangia, which, in some genera, may be 

 semisunken, and the individual sporangia of each pair remain distinct 

 at dehiscence, not fused as usual in the Magnoliaceae. (The stamens of 

 this family, especially those of Australian and East Indian genera, 

 should be studied for sporangial structure and position in the tissue of 

 the lamina. In MeJodora, Polyalthea, and Ancana, the connective is 

 described as "concealing the cells," suggesting that the sporangia are 

 completely sunken.) The Annonaceae may show, even better than the 

 Magnoliaceae, stages in the evolutionary transfer of the microsporangia 

 from a sunken to an apparently superficial ("protuberant") position. 

 Pollination is apparently largely by beetles, though beetles do not 

 feed extensively on the floral organs, as in Eupom-atia and Calijcantlnis. 

 The carpels are greatly reduced in number within the family, as are 

 the ovules — from numerous to one — but reduction in ovule number 

 does not accompany reduction in carpel number. 



The ovules are anatropous, as in most ranalian taxa, and reduction in 

 number from several to few parallels that in the Magnoliaceae. The 



