THE STAMEN 341 



showy, a mark for the visiting insect. In the great majority 

 of cases polHnation is carried out by insect agency or by 

 the wind. In wind-polHnated flowers calyx and corolla are 

 usually reduced or absent. We may first consider some 

 properties of the stamens and pollen. 



The Stamens.— The structure of the stamen or micro- 

 sporophyll is in ground plan very constant, though profound 

 modifications sometimes occur. Typically it consists of a 

 stalk, the filament, bearing a head, the anther. This has 

 four internal sporangia, elongated sacs which when ripe 

 are full of microspores, the pollen grains. At maturity 

 the anther opens, or dehisces, by two longitudinal slits, due 

 to the tearing of the external wall tissue between each two 

 sporangia away from the dividing wall ; the sporangia 

 thus open in pairs. The number of sporangia may be 

 increased by partitioning or decreased by abortion. The 

 form of the anther is subject to modification. The stamens 

 may be united more or less completely, or individual stamens 

 may be divided into numerous parts. Union of the stamens 

 with other floral organs, e.g. the style, may take place. 

 Staminodes are sterile stamens which may be rudimentary, 

 or may perform other functions. 



In the gymnosperms the structure of the microsporo- 

 phyll is much less uniform. In the Cycads very numerous 

 microsporangia are borne on the lower side of large micro- 

 sporophylls, which are, in Zamia, peltate. In Taxus about 

 half a dozen sporangia are borne on the lower surface of a 

 peltate scale. In Pinus two sporangia are borne on the 

 lower surface of a scale sporophyll. In Ginkgo the stalk- 

 like sporophyll bears two sporangia at its tip. In Welwit- 

 schia three, in Gnetum and Ephedra several sporangia are 

 borne on a stalk. In Ginkgo and the Gnetales the micro- 

 sporophyll is stamen-like, but in the other gymnosperms, 

 especially in the Cycads, the departure from the angio- 

 sperm type is very wide. The stamen, though strictly 

 homologous with the gymnospermous microsporophyll, is 

 so uniform in its general plan that it may be regarded as a 

 new type of organ, the stabilised end product of a long 



