THE ANGIOSPERMAE 1239 



which are broad and petaloid at the top, narrow downwards until in cross- 

 section they are flattened triangles, which ultimately become joined at 

 their edges to form the compound style. The apex of each triangle is 

 occupied by a furrow, with overlapping margins, and in the early stages of 

 development the whole structure has this form, resembling the funnels of 

 Crocus, with their inrolled margins. As development proceeds, the free 

 edges of the upper portions extend greatly, right and left, and become 

 coloured like the perianth. The two margins of the furrow also grow 

 upwards, overtopping the apex, which becomes the stigma, and forming a 

 pair of upwardly directed wings. The stigma in the mature structure thus 

 has the appearance of being merely a narrow flap on the abaxial side of the 

 stvle-branch. The upper end of the furrow, which is on the adaxial side, 

 lies between the two wings and close to the stigma, and it is by this channel 

 that the pollen tubes grow downwards to the inferior ovary. 



Placental stigmas are characteristic of the Ericaceae and are found 

 occasionally in other coenocarpous families. The compound style has a 

 central canal into which the placentae extend upwards from the loculi to the 

 apex, where they form the stigmatic lobes. Naturally they are sterile in 

 their upper portions. They have in this way replaced the carpel apices as 

 stigma-formers, and have made conducting tissue needless, since they 

 themselves function as conductors of the pollen tubes. The vascular 

 bundles of the style supply these placental stigmas, opening out into a 

 crown of storage tracheids below the stigmatic surface. Considering the 

 close association of placentae with the carpel margins, which so frequently 

 take part in the formation of both stigmas and styles, it is surprising that 

 placental stigmas do not often occur in more primitive families than the sym- 

 petalous Ericaceae, and one is tempted to wonder if perhaps it is not an 

 ancient feature which has been lost along the majority of evolutionary 

 lines. 



Sensitive stigmas are of occasional occurrence. Examples are Mimulns 

 and Martynia, in both of which there is a bilobed stigma, the lobes of which 

 are normally wide apart. On being touched they clap together suddenly 

 and thus entrap any pollen which may be carried by the visiting insect 

 which has touched them. 



Lastly we must mention the vexed question of the commissural stigma, 

 that is to say the stigma which stands over the line of carpel suture in a 

 coenocarpous ovary and not over the loculus as is usual. Some ot the best 

 known examples are to be found among the Papaveraceae and the Cruciferae, 

 in which families it is one of the outstanding characters. The usual explana- 

 tion of the peculiarity, with reference to the Cruciferae, is that the median 

 portion of the formerly dorsal stigmas has lost its receptive surface and that 

 the receptive margins have fused together with their neighbours, over the 

 line of suture. Each commissural stigma is therefore produced by the union 

 of two stigmatic margins. According to Arber the stigmas in Papaver arise 

 rather differently. The upper part of each placental wall, itself, as we have 

 mentioned above, a double structure, divides, and the two free surfaces 



