38 SEX AND HEREDITY 



There is also another point to be considered. Within 

 certain limits a difference of origin of the fusing gametes 

 is an advantage. Already in Ulothrix and in Ectocarpus 

 the conjugating gametes have been seen to arise from 

 different sporangia. In Flowering Plants it has been 

 shown in many cases that intercrossing gives on the 

 average a larger and stronger progeny. By crosssing 

 is meant that the pollen which produces the fertilizing 

 gamete shall have been derived from a flower or plant 

 distinct from that bearing the ovum which is to be ferti- 

 lized. Seeds that result from such a crossing have been 

 found to be on the average more numerous and heavier 

 than those resulting from self-fertilization. The course 

 of Evolution of Flowers has been such as to secure this 

 advantage. The effect of the separation of stamens and 

 carpels in space, and in time of maturing, is to promote 

 intercrossing. But all such developments have still 

 further complicated the mechanical problem of Polli- 

 nation for Seed-Plants. 



Such Plants being themselves immobile, as naturally 

 follows from their being rooted in the soil, use is made of 

 outside agencies, such as the movements of Wind and 

 Water, or the mobility of Animals. The mechanism of 

 flowers has been specialised in the most remarkable 

 manner in accordance with these methods of transfer. 

 Where use is made of Wind, as in the Grasses, the 

 flowers produce abundance of dry dusty pollen, easily 

 shaken out in clouds from anthers balanced on very 

 flexible filaments. The stigmas meanwhile are much 

 branched and feathery, so as to expose a large surface for 

 catching the grains. These features go with close 

 grouping of the flowers, which are individually small and 

 inconspicuous (Fig. 19). Where animals are the active 



