832 HOW PLANTS LIVE AND WORK. 



cone, the grains slide down the smooth scales, and very likely 

 come in contact with the ovules at the bottom. Each ovule has a 

 sticky drop of gum at the end, and the pollen is caught. Such 

 pollen grains as roll off the upper scales are almost certain to fall 

 on a lower one and reach its ovules. The pollen grains of these 

 trees are rendered particularly buoyant by being blown out at the 

 sides into little air-filled bladders. Not all wind-fertilised plants 

 have their male and female organs in separate flowers, however. 

 The Grasses afford a case in point. They have small and 

 (because they do not need to attract insects) insignificant flowers. 

 The stamens hang out loosely to the wind, and as the delicate 

 stem of the plant bends gracefully before every passing breeze 

 the pollen is readily detached and carried away to other flowers. 

 In order to catch the wind-borne pollen grains, the stigmas of 

 Grasses are generally branched and feathery. 



It is a practically invariable rule that no wind-fertilised flowers 

 are brightly coloured or conspicuous. 



The contrivances of insect-fertilised plants to prevent self- 

 fertilisation are of two chief kinds. In the first of these, the 

 male and the female organs of the flower ripen at different periods. 

 Generally, the stamens come to maturity first, and whilst the pistil 

 is yet undeveloped. A flower in this condition is to all intents 

 and purposes male only. When the stamens have shed their 

 pollen, and, therefore, finished their work, the pistil ripens, and 

 the flower is then female only. Occasionally there is an inter- 

 mediate period, however, when both sexes are represented together. 

 The second method adopted is for the plant to produce two, or 

 occasionally three, distinct kinds of flowers. 



A plant whose flowers afi^ord an excellent example of the first 

 of these devices is TropcBolmn^ which, under the name of " Nas- 

 turtium," is so common in our gardens. The honey of the flower 

 is contained in a long tube, the " spur." When the flower first 

 opens, neither the stamens nor the pistil are mature. As the 

 stamens ripen, one by one, the anthers turn up so as to stand in 

 front of the opening of the honey-tube, and a humble bee cannot 

 possibly reach the honey without brushing its breast against the 

 anther and carrying off some of the pollen. When all the pollen 

 is shed the stamens die, and the pistil turns up so that its stigma 



