THE STUDY OF FLOWERS. 



231 



STIGMA. 



There is thus here an arrangement preventing self- as well 

 as one ensuring cross-pollination (Fig. 96). 



Other examples which will reward study are Columbine, 

 Monkshood, Larkspur, and White Dead-nettle (Fig. 97). 



Arrangements preventing self-pollination are frequent. 

 Some of these are here enumerated, and it is suggested 

 that pupils be given opportunities for examining illustra- 

 tive types. 



The most familiar illustration is that in which stamens 

 and stigma mature at different times. The commonest 

 case is that in which sta- 

 mens ripen earliest (pro- 

 tandry), e.g. Harebell, 

 Crane's Bill, Compositae, 

 Umbelliferae, etc. If a 

 series of buttercups be 

 examined some will be 

 found whose outer stamens 

 have shed their pollen, 

 whilst the inner ones are 

 still curved inwards to- 

 wards the carpels which 

 are not yet mature. In 

 this case the carpels gener- 

 ally ripen as soon as the 

 inner stamens, so that 

 there is an overlapping 

 period when self-fertilisa- 

 tion is possible. 



There are some interesting cases in which the stigmas 

 of a flower ripen before the stamens (protogyny). Plan- 

 tains (Plantago) are a good example of this (although, 

 except in some special cases, this is not an insect-pollinated 

 flower). If the spikes are examined when their lower parts 

 show the protruding stamens with their pendulous anthers, 

 it will be seen that the stigmas in this region are already 

 withered, whilst in the upper parts the stigmas are mature 

 and the stamens are not yet extruded (Fig. 98). 



The Figwort (Scrophularia nodosa) is another interest- 

 ing case. In the young flowers the stigma may be seen 



BRACT 



Fig. 98. 

 A, Spike. B, Flower of Plaintain. 



