POLLINATION AND FERTILISATION 



>05 



tropic (p. 1 60). These three factors are effective in deciding the 

 course of the tube when it germinates normally upon the stigma. 

 They lead it to apply itself closely to the surface cells. 



On germination the contents of the pollen-grain pass over into 

 the growing tube. The nucleus of the vegetative cell with its cyto- 

 plasm usually passes out first, while the 

 antheridial-mother-cell is embedded in the rear- 

 ward part of the vegetative cytoplasm. It soon 

 divides to form two gametes, the nuclei of which 

 follow the vegetative nucleus (Fig. 226). As 

 the tube lengthens, the grain as well as the 

 older part of the tube is thus emptied of its con- 

 tents. Successive lengths are then shut off from 

 the distal part of the tube that is still full, bv 



\, J Fig. 225. 

 plugs of Cellulose, SO that as the tube advances Transverse section of the 

 •4- - „„i. - it 'ii - i. T-i style of Rhododendron, show- 

 it IS Still possible tO preserve ltS turgor. Thus ing the five-rayed channel 



provided, the tube can advance through long S whh SBST ut 

 distances to reach the ovule. (Compare S^S^S 



Fip" 22 I ^ ^ ots ' n the sect ^ on - (F. O- B.) 



From the surface of the stigma, the pollen-tube, under the com- 

 bined influence of its negative aerotropism, positive hydrotropism, and 

 positive chemotropism, is brought into close relation with the moist 



stigmatic tissues. Where there 

 is an open channel the pollen- 

 tube does not need to penetrate 

 the tissue. Even where, as in 

 Lily or Rhododendron, the 

 channel is filled with mucilage 

 the tubes penetrate the secre- 

 tion, but not the cells which pro- 

 duce it (Fig. 225). There is little 

 apparent difference in those 

 cases where, as in Salvia, there is 

 conducting tissue with mucilagi- 

 nous walls (Fig. 215) ; for there 

 the pollen-tubes penetrate the 

 mucilaginous middle lamella, 

 passing between the cells them- 

 selves. This is in fact the 

 commonest w r ay for the tube 

 to enter the tissue of the stigma, 



Fig. 226. 



A, Pollen-tube of Orchis latifolia teased out from 

 the ovary. t/ = vegetative nucleus, g, g = gametes. 

 ( x 500.) B, pollen-tube of the same penetrating 

 the micropyle : its gametes still in the tube. The 

 two synergids and the ovum (shaded) are clearly 

 shown. ( x 300.) (After Strasburger.) 



B.B. U 



