246 FLOWERS AND THEIR WORK. 



by insects. Such flowers are chiefly pollinated by the 

 wind. 



In a few water-plants the pollen is carried by water, and 

 pollination occurs at or below the surface, but this is rare ; 

 most aquatic plants raise their flowers well out of the water, 

 and are pollinated by wind (e.g. Pondweed) or by insects 

 (e.g. Water Crowfoot, Water Lilies). 



In wind-pollination it is obvious that the chances of a 

 pollen-grain striking a stigma are very small, and that the 

 plant must therefore produce a much larger amount of pollen 

 than is the case in insect-pollination. In order to increase 

 the chances of pollination the pollen is light, and can there- 

 fore float for a considerable time in the air and be carried to 

 a great distance, while the stigmas are usually large and 

 branched, to increase the amount of their surface. 



The anthers are usually carried on long dangling filaments ; 

 all the stamens of a flower open simultaneously, instead of 

 successively as in most insect-pollinated flowers ; and they 

 either open only in dry weather or are well protected (by 

 catkin -scales, bracts, etc.) against rain. 



In many wind-pollinated trees the inevitable waste of pollen 

 is to some extent reduced by the flowers opening before the 

 leaves have unfolded, or before they have grown large enough 

 to form a serious obstacle to the wind-carried pollen. In 

 most herbaceous plants with wind-pollinated flowers (e.g. 

 Plantains, Grasses, Salad Burnet, Docks, Sorrels) the latter 

 are carried up on a long stalk, well above the leaves, so as to 

 expose them as freely as possible to the wind. It will be 

 noticed that many wind-pollinated plants have the stamens 

 and the pistil in separate flowers, either on the same plant or 

 on separate plants, and that when they are both present in 

 the same flower the stigma nearly always matures before the 

 anthers open ; most Grasses, however, are protandrous. 



From these details and your own observations, draw up a 

 tabular comparison of entomophilous (insect-pollinated) and 

 anemophilous (wind-pollinated) flowers. 



274. Comparison of Flowers of Willow and Poplar. Ex- 

 amine and compare the flowers of Willow and of Poplar, which are 

 very closely allied plants. In both cases the male and female flowers 

 are on separate plants, and are carried on catkins, each catkin consist- 

 ing of an axis bearing numerous flowers and each flower standing in 



