468 ANGIOSPERMS less. 



Imagine one or more of the marginal lobes of such a leaf to 

 be replaced by megasporangia, as in Cycas (Fig. 122, e), 

 and the edges of its proximal part to come together and 

 unite (Fig. 126, b\ b^). The result will be the enclosure 

 of the ovules in a capsule formed from the proximal part of 

 the leaf, while its distal end forms the style and stigma. 



The extreme differentiation of both male and female 

 sporophylls is not the only important difference between the 

 angiospermous and the gymnospermous flower. Almost 

 equally characteristic, and equally striking as a sign of 

 advance in organisation, is the fact that the sporophylls are 

 surrounded by two sets — sometimes reduced to one — of 

 floral organs, the sepals and petals, which together form the 

 floral envelope or perianth. In most Gymnosperms the 

 only indication of a perianth is in the form of inconspicuous 

 oarren scales, />., scales not bearing sporangia, at the base 

 of the cone (Fig. 119, a and k, per), while in Angiosperms 

 the perianth has become differentiated into two well-marked 

 and conspicuous sets of leaves. 



The function of the sepals is usually to protect the other 

 parts of the flower in the bud : they are generally of such a 

 size as completely to close over the petals, stamens, and 

 carpels until the flower opens, when they often either turn 

 back or fall off. They are therefore to be looked upon as 

 leaves which have been modified for protective purposes. 



The petals serve an entirely different function. They are 

 usually large and brightly coloured, forming the most con- 

 spicuous part of the flower : they are also commonly scented, 

 and from them or some adjacent part nectar is secreted. 

 This fluid forms the staple food of many insects, especially 

 butterflies, moths, and bees, which, as soon as a flower is 

 opened, may be seen to visit it and to insert head or 

 proboscis in order to suck the sweet juice. 



