REPRODUCTION. 



227 



extine. The extine is firm and ciiticularized ; tlie intine is soft, 

 and consists of cellulose. The latter enters into the formation 

 of the pollen-tube. The points at which the pollen-tnbe forma- 

 tion is to begin are predetermined. The extine (Fig. 154, e) is 

 supplied with one (monocotyledons), several, or vmwj (dicotyledons) 

 open or thin areas (o, a). In some cases there is a lid-like cover 

 to these openings, which is removed when the pollen-tube begins to 

 develop. The intine ii) is usually thickened at these thin areas (see 

 Fig, 1 54). The protuberances and spines which sometimes occur on 

 the extine may serve to attach the pollen to insects as well as to the 

 stigma. The pollen-grains result from the quadrature of the pollen 

 mother-cells, which are known as tetrads in the first stage. Fig, 

 155 represents an early stage of the pollen-forming anther-case. 

 The cells immediately sur- 

 rounding the pollen mother- 

 cells, {eiS) as well as a layer 

 external to these, are subse- 

 quently dissolved. The cells 

 iep) are known as tapetal 

 cells. Between the stage 

 represented in the figure 

 and the mature stage the 

 young pollen-cells are found 

 floating in the granular liquid 

 which fills the entire anther- 

 case. (Studied more in par- 

 ticular by jSTageli, Hof- 



MEISTER, and "VVAEMINa). 



The ripe pollen-grains usu- 

 ally form a powdery mass. Orchids offer a peculiar exception, 

 the ripe pollen-grains of the entire anther remain united in a single 

 mass, forming the 2>ollinium. The teleology of the fertilization of 

 orchids has been made a special study by Darwin, 



Fig. 155. — Cross-section through a young 

 antbercase of Funkia cordata. 



sin. Pollen mother-cells ; ep, tapetal layer ; iv, epi- 

 dermis. (After Sachs.) 



C, The Gyncecium. The Ovule with the Embryo-sac 



BEFORE AND APTER FERTILIZATION. 



The gyncecium (pistil, according to the older terminology) bears 

 the ovules (seed- buds) in the lower hollow portion, the ovary. The 



