ii82 A TEXTBOOK OF THEORETICAL BOTANY 



be formed in sequence (Fig. 1154). See also under dimorphic flowers, 

 Chapter XXIV, p. 1275. 



The anther is the name appHed to the fertile portion of the stamen 

 when it is distinct from the filament. The stamens of primitive families, 

 like the Magnoliaceae, are usually undiff'erentiated, the pollen-sacs, which 

 are long and narrow, being attached to one side of or immersed in a strap- 

 shaped structure in which filament and anther cannot be distinguished. 

 The great majority of stamens, however, have a stalk-like filament, to the 

 distal end of which is attached a distinct anther portion, comprising a 

 central tissue, the connective, which supports and serves to unite the 

 pollen-sacs, sometimes called thecae. Normally there are four of these 

 sacs, in two pairs, one pair on each side of the connective, but at maturity 

 the members of each pair become united by the rupture of the wall 

 between them, so that when the anther finally opens to discharge the pollen 

 it may be correctly called bilocular. 



The traditional idea of a stamen equates it to a microsporophyll (Fig. 



1 155) and some discussion has 

 arisen on the question whether, 

 from this standpoint, the pollen- 

 sacs, or microsporangia, are 

 marginal or superficial. The 

 difference has been shown to be 

 important in distinguishing large 

 groups among the Pteridophyta 

 and might therefore be of some 

 evolutionr '■y significance. We 

 shall return to the evolution of 

 stamens later, but we must re- 

 mark here that it would be a 

 great mistake to assume that 

 characters manifested by Pteri- 

 dophyta or Gymnospermae 

 must also be present in Angi- 

 ospermae. The above question 

 also begs the further question 

 whether the stamen is, in fact, 

 a dorsiventral structure. The 

 stamen primordium, before the 

 appearance of the pollen-sacs, 

 is four-angled in transverse 

 section, a pollen-sac being 

 formed from each angle. Such 

 T^. .,, a structure is unifacial, not 



1 155- — UiiiKrams illustrating various tvpes , . 



of microsporangial branches. A, Gnet'ales. dorsiventral, and, unless we are 



B, Angiospermae. C Taxus. D, Coniferales. to regard it as analogous to a 



h, Bennettitales. F, Cordaitales. G, Rlivnio. .° , ^ . '^ , .. 



IF, Pteridospermae. {After Zimmermauu) Crucitorm leat SUCh as that ot 



Fig 



