FLOWERING PLANTS INTRODUCTORY 103 



dusted with pollen, and, while they may rub some of this on the 

 stigmas of the same flower, they are more likely, on going to 

 another flower, to effect cross-pollination. Cross-pollination is 

 often more advantageous, and many of the arrangements found 

 in flowers appear to render it likely or to prevent self-pollination. 



After pollination has been effected the sepals, petals, and 

 stamens of the Buttercup wither and fall off, but the group of 

 carpels remains on the receptacle, and enlarges to form the 

 fruit. Before considering this, however, it will be useful, as a 

 preliminary to the study of particular examples, to form some 

 idea of the chief modifications which are found in flowers. 



The Buttercup is a particularly good flower to start with, for 

 all its floral -leaves are inserted distinct ^ -^ ^ 

 from one another on the receptacle. In (._\(\f\(\ 

 most flowers, however, the distinctness of 

 the parts is modified by their union in 

 various ways and degrees. Examples will ^^ ^^^ ^.^ ^^^ 

 be met with in the plants described later. ( \( ]( ]( 

 Here it is only necessary to make clear 

 the nature of the modifications to be ex- 

 pected. In the first place, parts of the 

 same kind inserted at the same level on 

 the receptacle, i.e. belonging to the same 

 whorl, may be more or less completely FIG. 54. Diagram to iiius- 

 united together. When the sepals are trate the mode of union of 



..,.,,. , parts of the same whorl. 



joined in this way the calyx appears as 



a tubular or bell-shaped structure. This has usually a number 

 of free teeth or lobes projecting from the margin and indicating 

 the number of the united sepals. The petals are even more 

 commonly united to form a tubular corolla, and the stamens 

 are sometimes joined together in the same way. The union in 

 all these cases is not to be looked upon as a joining together of 

 distinct and separate, fully grown parts. It results from a modi- 

 fication in the development of the flower, which the diagram 

 in Fig. 54 will make clear. The petals, for example, arise as little 

 outgrowths of the receptacle. When they are distinct the further 

 growth is limited to the base of the outgrowth. But when growth 

 spreads to the region of the receptacle between the petals these 



