228 THE STRUCTURE OF FLOWERS 



threads (as in CEnothera), so as not to be readily dispersed in the 

 air, but to have some slight coherence as well as capability of 

 adhering to the head, limbs, or bodies of insects, especially to their 

 rough surfaces ; and in two families {OnhidacecB and Asdepiadaceci) 

 the pollen is combined in masses and with special adaptations for 

 being transported e7i masse. With this the stigma is usually 

 correlated, by roughness, moisture^or glutinosity." Sprengel was the 

 first to discover that in many species where the stamens and pistils 

 are situated in the same flower they do not mature at the same 

 time ; consequently the pollen cannot fertilise the stigma. Some- 

 times, as in the Arum, the pistil matures before the anthers. Such 

 plants are called proterogynous (or protogynous). In others the 

 anthers mature before the pistil. These plants are named proter- 

 androus (or protandrous). The familiar Arum moaelatum — (Plate 

 XXIV., Fig. 2) — the common arum or lords and ladies — -of our 

 woods and hedges is a good example of a proterogynous plant. 

 The well-known green leaf encloses a central pillar which supports 

 a number of stigmas near the base, and of anthers somewhat 

 higher. Nothing would seem easier at first sight than that the 

 pollen of the anthers should fall on and fertilise the pistils below 

 them. But this does not take place. The stigmas mature before 

 the anthers, and by the time the pollen has fallen have become 

 incapable of fertihsation. It is consequently impossible for the 

 plant to fertilise itself. Owing to the construction of the spathe 

 the pollen cannot be carried away by the wind. What happens is 

 as follows : The pollen when shed drops to the bottom of the tube, 

 where it remains secure from disturbance by wind. Small insects 

 attracted by the showy central spadix, or the prospect of honey or 

 shelter, enter the tube while the stigmas are mature. Above the 

 anthers and growing from the central pillar is a fringe of hairs point- 

 ing downwards. This contrivance allows small insects to enter, but 

 effectually prevents their exit until the stigmas have matured. After 

 a while, the stigmas have ripened and each secretes a drop of honey, 

 thereby rewarding the insects for their imprisonment. Then the 

 anthers ripen and shed their pollen, which falls upon and dusts the 

 insects. Shortly after the hairs referred to shrivel up and the insects 

 are set free. They carry the pollen with them, and on their visit 

 to another plant can hardly fail to deposit some of it on the stigmas. 



