326 POLLINATION AND FERTILISATION 



supplied by two vascular strands from the sporophyll. These 

 fork at its base to form two systems of branches, extending 

 almost to the extreme tip: those of the outer series (o.b.) traverse 

 the peripheral layers of the integument, whilst those of the 

 inner (i.b.) run close to the line of junction of the latter and 

 the nucellus. 



The wind- or insect-borne microspores (pollen grains) are 

 drawn into the pollen chamber by the drying up and contraction 

 of the mucilaginous fluid (formed by the disintegrating cells of 

 the nucellus) which exudes from the micropyle at the time of 

 pollination. Within the pollen chamber germination ensues 

 (Fig. 186, C, and 187, C), and a short branching sucker-like 

 pollen tube (p.t.) grows into the adjacent tissue of the nucellus. 1 

 Subsequently two spermatozoids, each with a spiral band of 

 cilia 2 (Fig. 187, B, s.), are developed within the main body of 

 the microspore. In the meantime the' large megaspore has 

 become filled with a uniform tissue, the female prothallus (Figs. 

 186, C, and 187, C, p.) which produces a, commonly small, number 

 of archegonia (ar.), with very minute necks (n.) and large eggs (o.), 

 at the end adjacent to the micropyle. 



The nucellar tissue between the pollen chamber and the 

 female prothallus breaks down (Fig. 187, C), with the formation 

 of a slimy fluid in which the liberated spermatozoids (s.) swim 

 to the archegonia (o.), and in this way fertilisation is accom- 

 plished. The nucleus of the fertilised egg divides repeatedly 

 to form numerous nuclei which, at least in the lower part of 

 the oospore, become separated by cell- walls. It is this region 

 alone that 'gives rise to the embryo (Fig. 187, D, e.), whilst the 

 remainder serves for nutrition. 



In each ovule only one embryo ultimately develops, its growth 

 taking place at the expense of the surrounding prothallus, into 

 the centre of which it is carried by the marked elongation of a 

 suspensor (Fig. 187, D, sp.). Upon reaching a certain stage, 

 however, in which two cotyledons, plumule, and radicle can be 

 distinguished, the embryo becomes dormant, the residue of the 



1 The sequence of events in the germination of the microspores of 

 Cycads is very similar to that in Conifers (cf. p. 348). 



2 Apart from the Cycads, the Maiden-hair Tree (Ginkgo biloba) is the 

 only seed-plant which has ciliated, free-swimming, sperms. 



