

I'.OTANY OF THE LIVING PLANT 



and it is well illustrated in the Grasses. Here the tubes force their 

 between the stigmatic cells, penetrating the middle lamella of 

 their walls. But occasionally the cells of the stigma are themselves 

 perforated. This is seen in the Corn Cockle (Agrostemma), where the 

 pollen-tube traverses the delicate cell-wall of the stigmatic papilla 

 (1-il;. 222, A). The protoplast of the perforated cell is not killed, and 

 it may even continue its movements for a time, and retain its tur- 

 ence. The tube passes out between the walls of the subjacent 

 conductine cells, and continues its course in that way. The Mallow 

 behaves similarly, with the further feature that a number of tubes are 

 formed from each of the large grains, serving to fix the grain on the 



surface of the stigma. One tube, 

 however, grows larger than the rest, 

 and conveys the essential contents 

 of the grain (Fig. 222, B). It thus 

 appears that pollen-tubes behave 

 upon the surface of the stigma like 

 the filaments of parasitic Fungi, 

 which similarly either follow the 

 surface of the invaded tissue or 

 grow between its cells ; but some- 

 times they penetrate the cells them- 

 selves. There is no doubt that in 

 its course the pollen-tube also 

 draws nourishment from the tissue 

 it traverses. 



Passing thus down the style and 

 into the cavity of the ovary, the 

 tube is often conducted mechani- 

 cally by directing hairs towards 

 the ovule, which in the common 

 inverted type has its micropyle 

 close to the wall of the ovary. 

 The last part of the course is 

 believed to be influenced by the 

 sSSbESS 8 ° f the embry °- sac - (From synergids ; in some cases a drop of 



fluid, derived perhaps from them, is 

 exuded from the micropyle. Whatever the influences may be, the tube 

 enters the micropyle and impinges closely on the apex of the nucellus ; 

 where that tissue has already been absorbed, it may advance directly 

 upon the embryo-sac, close to the egg-apparatus (Fig. 226, B). 



Fig. 227. 



A. embryo-sac of Helianthus annuus (after 

 •ischin). B, the male nuclei more highly 

 magnified. ps = pollen-tube. s,s 2 =svnergidae. 

 *fa,*Pi male nuclei. ov = egg-cell. <?& = central 

 fusion-nucleus of embryo-sac. a = antipodal 

 cells. sp t fertilises the egg ; sp. 2 fuses with the 



