102 THE FLOWERING PLANT. 



area, the stigma, to which pollen may often be found adhering. 

 By splitting open the ovary, several little green ovules may be 

 found attached along its upper margin. Now examine a young 

 pea-pod, which is a further developed pistil. At one end the 

 stalk and calyx will be seen, at the other the remains of the 

 style, while the pod is the developed ovary. By holding it up 

 to the light, a row of matured ovules, i.e., seeds, will be observed 

 running along one side. Split open the pod on that side and 

 spread it out. It will then look like an ordinary leaf, with well- 

 marked midrib and thickened edges bearing ovules. Every 

 carpel of an apocarpous pistil is, in fact, regarded as a folded 

 leaf, the thickened edges of which bear ovules, and are united 

 together in a seam or suture, called the ventral suture. The 

 other edge of the carpel may be called the dorsal margin, which 

 corresponds to the midrib. The thickened edges here form the 

 'placenta, and the arrangement or placentation of the ovules is 

 here said to be marginal (fig. 48 D). In order to grasp the idea 

 that a pistil like that of the pea corresponds to a folded leaf, 

 take some simple leaf that tapers gradually to a point, e.g., one 

 from a fuchsia, and fold it upon the midrib ; you will then see 

 that the broader part answers to the ovary, and the narrower 

 part to the style. It sometimes happens in abnormal flowers 

 that carpels remain partly or entirely open, thus showing their 

 true nature, and the syncarpous pistil of mignonette never com- 

 pletely closes at the top (cf. also p. 107). In gymnosperms 

 folding has not taken place at all. We may consider that the 

 pea has descended from ancestors somewhat resembling the 

 gymnosperms of the present day, and that the formation of a 

 closed ovary has taken place gradually in the course of innu- 

 merable generations. The next question is, " Which surface of 

 the folded leaf is inside, upper or lower ? " This query cannot 

 easily be answered by reference to the pea alone, but examination 

 of apocarpous pistils with more than one carpel will readily 

 give a solution. Examine, for instance, a head of ripe fruit 

 in larkspur, columbine, or marsh-marigold, where three, five, 

 and several carpels are respectively present. It will be seen 

 that the ventral sutures face inwards, for in the fruit they will 

 have split open, allowing the seeds to be seen attached to their 

 edges. But facing inwards means facing the shortened axis or 

 receptacle, and since folding has taken place in this direction, 

 the upper surface of the leaf must bound the internal cavity. 

 Take a piece of stem with a foliage leaf attached, and fold this 

 up so that the approximated edges face the stem, and the nature 

 of the above process will be seen. In the single carpel of pea 

 the ventral suture faces upwards , and since there are five sepals, 



