The Present Flora of Britain. Il 
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 
