THE ANGIOSPERMAE 



1295 



number of unrelated genera of water plants. Callitriche autumnalis or 

 Rtippia inaritima serves to illustrate this mechanism. In both these types 

 the pollen grains are less dense than the water and float up to the surface 

 where they come into contact with those flowers which mature at the water 

 surface. 



In Ruppia mariiima the flowers are produced below the water and are 

 hermaphrodite but markedly protandrous (Fig. 1225). The flowers grow in 

 pairs on a common stalk. Each flower is entirely naked and consist of two 



Fig. 1225. — Ruppia niaritima. A, F"lo\vering shoot, showing the spiral peduncles of 

 the female flowers. B, Inflorescence with male flower above and female flower 

 below. (A after Butcher and Strudivick. B after Le Maout and Decaisne.) 



stamens and four carpels, arising from a spathe-like leaf-sheath. In the early 

 stages of development the flowers are functionally male and are extremely 

 short, scarcely projecting beyond the enveloping sheath. Each anther as it 

 matures discharges its pollen into the water. These pollen grains are light 

 and float t ) the surface and as in the last example are devoid of any extine. 

 Each grain is tubular in shape, but bent at a right angle. After the discharge 

 of the pollen the inflorescence functions as a pair of female flowers. Its stalk 

 elongates greatly and the carpels, which also develop long pedicels, are 

 carried up to the surface of the water, where the stigmas come into contact 

 with floating pollen, which is moved about on the surface by wind and 

 water currents. After pollination has occurred the stalk again contracts 

 I 



