166 THE STRUCTURE OF FLOWERS. 



taken place.* M. Guignard has described the effects result- 

 ing from his experiments. f Thus, in the case of Vanilla 

 aromatica, he found the development of the ovary was very 

 rapid after pollinisation. At the time of flowering, the 

 placentas have only the rudiments of the papillae which 

 will develop into ovules, and the couducting tissue formed 

 by the epidermis and subjacent layers on either side of 

 the placentary projections is still undifferentiated. In 

 the intervals which separate the bands of conducting tissue, 

 corresponding to the midribs of the carpels, there is no 

 appreciable modifications before fecundation ; but as soon 

 as that has taken place, a layer of elongated papillae, filled 

 with a granular substance, arises. With regard to the 

 development of ovules, M. Guignard remarks : " La pollinisa- 

 tion et la germination du pollen sont indispensables a leur 

 formation. L'ovaire d'une fleur non pollinisee ne s'accroit 

 pas et tombe quelques jours apres I'epanouissement." 



As soon, however, as the pollen-tubes are formed, the 

 ovules begin to grow, until the twentieth day, when the pri- 

 mine thickens (much more than in other orchids) and finally 

 gives to the matured ovule a globular form. 



In the mean time the embryo-sac and sexual apparatus 

 have been forming, and are completed (excepting the fusion 

 of the two members of each tetrad, which does not take place 

 to form the secondary embryo-sac nucleus) in little more 

 than a month after pollinisation. Five weeks after that 

 period, fecundation commences. 



In following the progress of the pollen-tubes, it is not 



* Die Fruchthildung der Orchideen, ein Beweis fiir doppelte Virhung 

 des Pollen, Bot. Zeit., 1863. Bastardirungsversuche an Orchideen, Bot. 

 Zeit., 1865. 



t Ann. des Sci. Nat., 1886, torn, iv., p. 202; see also Maury, 

 Observations sur la Pollinisations des Orchidees, comp. rend, de I'Acad. 

 des Sci., 2 Aout, 1886; and also Guignard, do., 19 Juillet, 1886. 



