The Present Flora of Britain. ii 



one of them, the plant cannot maintain itself. The seed 

 must have the right temperature, soil, and amount of 

 moisture to enable it to germinate and grow. The young 

 plant must have sufficient vigour to defend itself against 

 parasites or aggressors — not like the wheat which cannot 

 grow among our ordinary weeds, and depends on human 

 protection. The climatic conditions at the time of 

 flowering must be favourable, or the ovule may not be 

 fertilised. For the ripening of the seed a certain critical 

 temperature must be reached, and maintained for a 

 sufficient time. The cold or wet in the winter must not 

 be such as to destroy the seed before it has germinated. 

 All these conditions must be favourable or the plant can- 

 not establish itself An annual plant must seed every 

 year, and go through the whole round safely, or it will be 

 destroyed. A perennial plant need seed and grow from 

 seedlings only once in a generation. 



As instances of what is meant by these remarks I will 

 take a few common plants. The horse-chestnut grows 

 well even as far north as Bergen in Norway, and in Britain 

 it produces abundance of ripe seeds every year ; but even 

 in the south of England, as far as I am aware, it never 

 succeeds in establishing itself from self-sown seeds. The 

 common elm {Ulmus campestris), on the other hand, in 

 England only produces perfect seed about once in forty 

 years. Forty years is far less than the lifetime of an elm, 

 and if the tree seeds once in a lifetime, and the seed 

 germinates, the species may establish itself. Perfect seeds 

 have not come under my observation, and I cannot there- 

 fore say whether this elm does grow from seedlings. It 

 is generally said only to occur where planted. The 

 butcher's broom {Ruscus aculeatus) is an instance of a 

 plant which just manages to hold its own. After watching 

 its fruiting for twelve years in succession, I find that as a 



