GROUP IT.- -PHANEEOGAM1A : ANGIOSPERM^E. 



527 



the andrcecimn and the corolla {e.g. Capparidaceae, Snpindaceae, 

 Resedaceae), or, less commonly, between the corolla and the calyx 

 {e.g. some Apocynaceas, such as Xerium) : in others it is intra- 

 staminal, that is, between the androecium and the gynseceum (as 

 in Rutaceae, Rhamnaceaa, Celastraceae, etc.). Again, the disc is 

 generally hypogynous, but sometimes epigynous (Umbelliferse). 



Generally speaking, when the nectaries, of whatever kind, are 

 towards the outside of the flower, the anthers are extrorse (e g. 

 Ranunculaceae); and when towards the centre of the flower, the 

 anthers are introrse. 



The most striking accessory structures are those of the Passion- 

 flower : these are coloured filaments, borne in successive whorls 

 (as many as five), between the corolla and the androecium: they 

 are not glandular, neither 

 can they be regarded as 

 staminodia or as modified 

 petals. 



The General Histology 

 of the sporophyte is suf- 

 ficiently treated of in Part 

 II., and in the general 

 account of the Phanero- 

 gams (p. 440). 



The Hmbryogeny of the 

 sporophyte is considered 

 on p. 440. 



The GametopTiyte is con- 

 sidered on p. 447. 



Fertilisation. After 

 reaching the stigma the 

 pollen-grains protrude the pollen-tubes which penetrate through 

 the tissue of the style into the cavity of the ovary, and through 

 the micropyle of each ovule to its nucellus (Fig. 339 P n). The 

 time required by the pollen-tube for this process depends partly on 

 the distance of the pollen-grain from the ovule and partly on the 

 specific peculiarities of the plant; thus the pollen-tube of the 

 Crocus takes only from one to three days to traverse the style, 

 which is from five to ten centimetres in length ; but in the Orchids, 

 where the length of the style varies from two to three millimetres, 

 several days, weeks, or even months are needed, and it is during 

 this process that the ovules are formed in the ovary. 



V, S. B. MM 



Fia. 339. Diagram of an ovule shortly after 

 fertilisation ; a outer, and i inner integuments ; / 

 f unicle : fc nucellus. S Embryo-sac in which E is 

 the embryo developed from the fertilised oosphere. 

 The sac also contains the endosperm-cells which are 

 being formed by free cell-formation. P The pollen- 

 tube, passing through the micropyle, n. 



