MORPHOLOGY OF EEPEODUCTIVE ORGANS 131 



of the edges of the carpel known as the placenta. Sometimes 

 the placenta is borne upon the axis. 



The true nature of the union of the carpels with each other 

 will be discussed in a succeeding section. 



It is customary to regard the carpels as sporophyUs, even in 

 those cases in which the megasporangia really spring from the 

 axis, which in these cases protrudes into the cavity of the ovary, 

 as in the Polygonaceae, already mentioned. Other instances of 

 this are found in the Primulaces, the Primrose family. 



Other structures are occasionally to be met with in certain 

 flowers. In some of the genera of the Boraginaceas and the 

 Caryophyllacese a scale or ligide is found at the junction of the 

 claw and the limb of the petals {figs. 308 and 327). In the Daffodil 

 (fig. 328) these scales or ligules are coherent together and form 

 a tubular structure known as the corona. Many flowers also 

 contain certain bodies which excrete a sugary liquid. These are 

 called nectaries. They have no definite position like the parts 

 already described, but may arise from the metamorphosis of 

 various structures. Certain foliaceous scale-like bodies known 

 as staminodes may also be found in some flowers. They may 

 be of various shapes and sizes. These are always derived from 

 the metamorphosis of a stamen. 



Symmetry of the Flower. 



The term ' symmetry ' has been variously understood by 

 different botanists. It was formerly usual to describe a flower 

 as symmetrical when all the whorls of its members have an 

 equal number of parts, or when the parts of one whorl are a 

 multiple of those of another. Such a flower is preferably to 

 be described as isomerous. Thus, in some species of Crassula 

 [fig. 245), we have a flower composed of five sepals, five petals, 

 five stamens, and five carpels ; in Sedum {fig. 246) we have 

 five sepals, five petals, ten stamens in two whorls, and five 

 carpels ; in the Flax we have five sepals, five petals, five 

 stamens, and five carpels, each of which is partially divided into 

 two by a spurious dissepiment {fig. 421) ; in Circcca {fig. 247) 

 we have two members in each whorl ; in the Rue {fig. 282) we 

 have four or five sepals, four or five petals, eight or ten stamens, 

 and a four- or five-lobed pistil ; and in the Iris there are three 

 members in each whorl. When the number of parts in each 

 whorl is not the same, or when the parts of a whorl are not a 

 multiple of those of another, the flower is heteromerous, as in 



