OVULE 



8S 



to the funicle (figs. 109, 112). (3) A campylotropous ovule is 

 one in which the nucellus is itself curved ; often it is kidney- 

 shaped (figs, no, 113). 



FERTILISATION AND CHANGES IN THE OVULE. 



When a healthy pollen-grain reaches a suitable stigma (fig. 

 1 14 sg) it germinates by sending a slender tube {pt) down the 

 style. The end of this pollen-tube eventually enters the 

 micropyle (w), and comes into contact with the nucellus 

 close to the top of the embryo-sac (es).* 



The consequence of fertilisation is 

 that the ovule grows and becomes a seed 

 (compare figs. 114 and 115). The most 

 important change in the ovule is that 

 a minute new plant — the embryo — 

 develops inside the embryo-sac. The 

 " embryo-sac grows and becomes, wholly 

 J. a; or partially, filled with endosperm, which 

 surrounds the 

 embryo. This 

 endosperm may 

 be present still 

 in the seed, and 



Fig. 114.— Vertical section ^^ ^eed IS Said p,f 



through a carpel and an anatro- tO be endospermic 

 pous ovule, showing the pollen- ^ A ti' 



tube entering the micropyle. ^S- VjraSS anu 



Violet (figs. 118, wi? 

 158). Or the endosperm formed in 

 the ovule may be gradually absorbed 

 by the growing embryo, so that in the 

 ripe seed there remains no trace of it : 



, ^>. "1,1 _7. i-"^ jDuiLeruup, Willi ui 



the seed is then said to be 7ion-enaosper- anatropous seed : per 

 mic-e.g. Bean and Wallflower (fig. 116). ^:^^,iJ:='^^^^^Z. 

 In most flowering plants, whilst the em- 

 bryo-sac, the contained endosperm, and the embryo are grow- 



* This process can only be followed properly by the aid of a compound 

 microscope. In a few Angiosperms the pollen-tube does not enter by 

 the micropyle, but pushes its way through the substance of the ovule till 

 it reaches the embryo-sac. In the Pine and Gymnosperms the pollen- 

 grain itself reaches the micropyle and then sends out a tube. 



emh 



Fig. 115. — Vertical sec- 

 tion through a fruit of 

 the Buttercup, with one 



