DEHISCENCE OF STAMENS 



363 



free of the pollen. Drying is usually due to direct loss of moisture 

 to the air, but may result from the withdrawal of water by 

 neighbouring cells of high osmotic content, belonging either to 

 nectaries (e.g. Stellaria) or to sugar-containing tissue (e.g. Fox- 

 glove, Digitalis) ; in the latter case dehiscence of the stamens 

 ensues, even in damp air. 



Actual splitting usually takes place longitudinally, along the 

 middle line of each anther- 

 lobe ; the line of weakness is 

 sometimes bounded on either 

 side by a spindle-shaped group 

 of enlarged epidermal cells (e.g. 

 Lily). The radial thickenings 

 of the cells of the fibrous layer 

 (Fig. 212, en.) prevent con- 

 traction in this direction, so 

 that the shrinkage during dry- 

 ing is mainly tangential, and it 

 is probably the tension thus set 

 up that finally results in rup- 

 ture. In a few flowers dehis- 

 cence takes place in other ways, 

 as, for instance, by apical pores 

 (Ericaceae), or by the forma- 

 tion of subapical valves which 

 hinge backwards to allow the 

 shedding of the pollen (e.g. Bar- 

 berry). 



Before the pollen grain is 

 shed its nucleus divides into two 

 (Fig. 215, B). One of these 

 becomes surrounded by a small 



envelope of denser cytoplasm (generative cell, g.c.), whilst the 

 other (lube nucleus, t.n.) lies freely in the general body of the 

 grain. In this condition the latter is transferred, in one way 

 or another, to the stigma of the same or of a different flower, 1 

 and here germination takes place (Fig. 215, F). The tube nucleus 

 passes into the tip of the pollen tube, and the naked generative 

 1 Cf. F. and S., chapters xx, xxi. 



en. 



m 



FIG. 212. Transverse section 

 through part of a pollen sac 

 of a ripe anther, en., fibrous 

 layer ; m., middle layers ; p.g., 

 poiien grains; t., disorganised 

 tapetum. 



