408 XXIX. THE GYN(ECIUM OF ANGIOSPERMS. 



largest possible proportion of the pollen- tubes. These are followed 

 here, with the greatest possible ease, up to the apex of the embryo- 

 sac, between the caps of the synergidae, and right up to the 

 oosphere (D, E). We see that the pollen- tubes, " conducted " by 

 the placentae, are still further guided by the funicles till they reach 

 the apex of the embryo-sac. A direct influence makes itself felt 

 at the same time from this latter, which determines the direction 

 of growth of the apex of the pollen-tube. It can be assumed that 

 the synergidae secrete some definite substance which acts chemo- 

 tactically as a stimulus upon the pollen-tube. The caps of the 

 synergidae, on account of their soft consistence, oppose little 

 : resistance to the escape of the secretion. Where the caps of the 

 synergidae are specially strongly developed, they appear, moreover, 

 traversed by very fine canals, which conduct the secreted sub- 

 stance outwards. The synergidae in Torenia, as elsewhere, be- 

 come disorganised after the entrance of the pollen-tube, and take 

 on the strongly-refractive appearance already known to us. -For 

 the study of the further developmental stages of the embryo, etc., 

 this object is not favourable. 



One of the easiest plants in which to see the passage of the 

 pollen-tube into the micropyle is a speedwell, Veronica serjjylli- 

 folia, very common in shaded grass and in hedgerows. Take 

 a flower from which the corolla has just fallen, place the ovary on 

 a slide in a drop of 3 per cent, sugar solution, and dissect out 

 the ovules under the microscope or under a simple microscope. 

 If the ovules are carefully separated, and a cover-glass put on, 

 some of them will probably be found to show the pollen-tube 

 in the micropyle. 



