88 ROUND THE YEAR 



buried and forgotten, and this is able to germinate. 

 A small seed would tempt no animal, but a large seed 

 protected by a hard shell is worth carrying off, and 

 yet has a chance of germinating after all. 



Why are the pods of the Willow soft and green, 

 while the cones of the Alder and Birch and the nuts 

 of the Hazel are woody ? Probably because the 

 minute seeds of the Willow ripen quickly and are 

 easily dispersed. They require no protection against 

 the rains and frost of winter, as the slow-maturing 

 seeds of Alder, Birch and Hazel do. 



One question more. Why are trees so often 

 completely unisexual ? Where many flowers are 

 borne upon one plant, as is commonly the case with 

 trees, they would infallibly fertilise one another 

 continually, if all were perfect. By the complete 

 separation of the stamens and pistils, self- fertilisation 

 becomes impossible. 



Annual plants are hardly ever completely unisexual. 

 The transport of the pollen from one plant to another, 

 whether by Insects or by the wind, is an operation 

 which might conceivably be hindered in a particular 

 year by deficiency of a particular species of Insect, by 

 perfectly still weather, or by long-continued rain. 

 Such accidents, even though they came round but 

 once a century, or once in a thousand years, would 

 greatly reduce the numbers of an annual plant, and 

 might even exterminate it. But it would signify 

 little to a tree that the whole crop of seeds should fail 

 in. a particular year. 1 



1 See Darwin's Cross- and Self -fertilisation of Plants, 

 Chapter X. 



