POLLINATION AND FERTILISATION 



269 



Fig. ^14. 

 Transverse section of the 

 style of Rhododendron, show- 

 ing the five-rayed channel 

 lined with cpitheliuiu, and 

 filled with mucilage : it is 

 traversed by the pollen-tubes, 

 which appear as compressed 

 dots in the sfctir.n. 



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 rearward part of the vegetative cytoplasm. It soon 

 divides to form two gametes, the nuclei of which ___ 



follow the vegetative nucleus (Fig. 213, A). 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, by 

 plugs of cellulose, so that as the tube advances 

 it is still possible to preserve its turgor. Thus 

 provided, the tube can advance through long 

 distances to reach the ovule. (Compare 

 Fig. 211.) 



Germinating on the surface of the stigma the 

 negative aerotropism, positive hydrotropism, 

 and positive chemotropism all lead the tube to a 

 close relation with its moist 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 withmucilagethe 

 tubes penetrate thesecretion, but 

 not the cells which produce it ( Fig. 

 214). There is little apparent 

 difference in those cases where, 

 as in Salvia, there is conducting 

 tissue with mucilaginous walls 

 (Fig. 205) ; for there the pollen- 

 tubes penetrate the mucilagin- 

 ous middle lamella, passing 

 between the cells themselves. 

 This is in fact the commonest 

 way for the tube to enter the 

 tissue of the stigma, and it is 

 well illustrated in the Grasses. 

 Here the tubes force their way 

 between the stigmatic cells, 

 penetrating their middle lamella. 

 But occasionally the cells of the stigma are themselves perforated. 

 This is seen in the Corn Cockle (Agrostemina), where the pollen-tube 

 traverses the dehcate cell-wall of the stigmatic papilla (Fig. 212. .-/). 



^- 



B Fig. 214 bis. A 



A, Pollen-tube of Orchis latifolia teased out from 

 the ovary. i» = vegetative nucleus, g, g = gametes. 

 ( > 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.) 



