202 



PLANT STRUCTURES 



ordinary nutritive cells of the gametophyte remains, and 

 the whole structure seems to represent a single antherid- 

 ium. At first it consists of two cells, the large wall cell 

 and the small free generative cell (Fig. 172, D). Later 



the generative cell di- 

 vides (Fig. 172, E), 

 either while in the 

 pollen -grain or after 

 entrance into the pol- 

 len-tube, and two male 

 cells (sperm mother- 

 cells) are formed (Fig. 

 172, F), which do not 

 organize sperms, but 

 which function direct- 

 ly as gametes. 



When pollination 

 occurs, and the pollen 

 has been transferred 

 from the pollen-sacs 

 to the stigma, it is de- 

 tained by the minute 

 papillae of the stig- 

 matic surface, which 

 also excretes a sweet- 

 ish sticky fluid. This 

 fluid is a nutrient so- 

 lution for the micro- 

 spores, which begin to 

 put out their tubes. 

 A pollen-tube pene- 

 trates through the 



FIG. 173. Diagram of a longitudinal section through 

 a carpel, to illustrate fertilization with all parts 

 in place : s, stigma ; g. style ; o, ovary ; ai, ii, 

 outer and inner integuments; n, base of nucel- 

 lus ; /, funiculus ; b, antipodal cells ; c, endo- 

 sperm nucleus; k, egg and one synergid; p, pol- 

 len-tube, having grown from stigma and passed 

 through the micropyle (m) to the egg.-After st i gmat i c sur f ace , en- 



LUERSSEN. > 



ters among the tissues 



of the style, which is sometimes very long, slowly or rap- 

 idly traverses the length of the style supplied with food by 



