324 APPENDIX II 



The central structure of the flower is the female organ, 

 which is known as the pistil or gynwceum. As the androecium 

 is made up of stamens, so the gynasceum is made up of one or 

 more carpels, either free or more often fused together. Each 

 carpel may be regarded as essentially a leaf, bearing immature 

 seeds, or ovules as they are called, along its margin. Its nature 

 is, however, obscured by the fact that the edges of the leaf 

 are fused together, so as to enclose the ovules. The structure 

 of a pistil, consisting of a single carpel, can be best understood 

 by splitting open a pea-pod, down the edge along which the 

 peas are attached. The pod, when opened out, is seen to be 

 only slightly changed from a leaf folded along its midrib. 



The part of the carpel which contains the ovules is called 

 the ovary. It is prolonged upwards, as a columnar or thread- 

 like outgrowth, called the style, which terminates in a special 

 portion intended for the reception of the pollen grains, and 

 known as the stigma. The stigma is frequently hairy or sticky, 

 so as more easily to catch and retain the pollen grains. 



A better crop of seeds is usually produced when pollen 

 from another flower is deposited on the stigma — or in other 

 words, when cross-fertilisation, and not self-fertilisation, takes 

 place. Cross-pollination is brought about either by the 

 arrival of pollen blo\vn by the wind, or brought by insect 

 visitors which, attracted by the coloured perianth or by a sweet 

 scent and a prospect of honey, fly from flower to flower, and 

 thus unintentionally convey the pollen of one flower to the 

 stigma of another. 



When the pollen-grain reaches the receptive surface of the 

 stigma, it grows out into a long slender tube (only visible 

 under the microscope), which travels down the tissues of the 

 style into the cavity of the ovary, and, advancing towards one 

 of the ovules, enters it by a tiny aperture in its outer coat. 

 The male fertilising element, or sperm, passes down the tube, 

 enters the ovule, and fuses with a cell within it known as the 



