THE SYCAMORE ''KEY" 



nectar ; some of these flowers bear only pollen- 

 producing stamens, but some have gone farther 

 and produced in addition a central ovary, pro- 

 tected with tiny hairs, in which seeds were to 

 be matured after the pollen-dusted fly had paid 

 a visit ; for when the stigma is pollinated fer- 

 tihsation begins, and so the seeds become per- 

 fected and continue their development. 



These devices for attracting fly visitors the 

 plant has slowly evolved through having derived 

 benefits from their incidental visits, and the 

 principle of producing some flowers that have no 

 ovary, and others whose stamens never come 

 to maturity, together with flowers which mature 

 both stamens and ovary, is a very economical 

 one. It insures that some flowers, at all events, 

 will be cross-fertilised instead of being fertilised 

 from the pollen of their own flowers. In the 

 course of time it is quite possible, I think, that 

 the sycamore will dispense altogether with flowers 

 that produce both stamens and ovary together, 

 and so make all its flowers distinctly male or 

 female, and thus insure cross-fertilisation for all 

 its seeds. Cross-fertilisation, as has been already 

 observed, produces much stronger offspring than 

 self-fertilisation. 



The sycamore therefore presents an exceed- 

 ingly interesting example of a tree slowly evolving 

 flowers of one sex only, or what the botanist terms 

 unisexual flowers. Many trees have completed 

 their evolution in this direction, for example, oaks, 

 alders, hazel, etc., while some, as the willows, have 



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