98
THE ESSEX NATURALIST.
The thanks of the meeting were accorded to Mr. Whitehead for his
communication.
The President then called upon Miss Winifred E. Brenchley, D.Sc.,
Botanist to the Rothamsted Experimental Station, who gave a lecture
entitled "Weeds and their relation to Soils and Crops," which she illus-
trated by an excellent series of lantern slides. Dr. Brenchley kindly fur-
nishes the following resume of her address:—
WEEDS AND THEIR RELATION TO SOILS AND CROPS.
A weed on arable land may be defined as any plant other than the
crop sown, but on grass-land it may be either a plant of low feeding value
or one that grows so luxuriantly as to crowd out more valuable species.
Weeds do damage by robbing crops of food, water and light, and
so reducing the yield, by adding expense to cultivation, by spoiling wool
and seeds, and, in the case of parasitic weeds like dodder, yellow rattle and
broomrape, by actually enfeebling or destroying the crop plants which
are used as hosts. The weed problem is of such salient importance to
agriculture that in many parts of the world active legislation is directed
towards the destruction of weeds, and in this country a Seed Testing
Station has recently been established at Cambridge to ensure the purity
of the crop seeds sown.
If weeds are left unchecked they will crowd out the crop entirely, and
eventually the vegetation will revert to the natural condition for the area.
Half an acre of wheat left unharvested in 1882 at Rothamsted was covered
with a dense weed-flora four years later, the wheat having almost entirely
disappeared, and now the area has reverted to woodland. The crop
plants are usually alien and depend upon man's assistance in order to flour-
ish, they are therefore unable to withstand the keen competition of the
weeds, which are usually native or very adaptable to conditions.
Weeds are readily distributed by the agency of wind, animals, manure,
farm implements, etc., and in addition the seeds are often able to maintain
their vitality for long periods when buried in the soil. Experiments have
shown that after 32 years burial no less than three million arable weed-
seeds can survive per acre, and after 10 years burial as many as 19 million!
Even after being eaten by animals and birds and voided in excreta large
numbers of weed-seeds survive uninjured.
Many weeds have been transferred from one part of the world to
another, especially the commonest and most harmful weeds, which are
very adaptable to various conditions of soil and climate.
For some years past a weed survey has been carried out from Rotham-
sted to determine the associations of weeds with particular soils and crops.
A preliminary classification has been made for arable weeds, but this
may need modification as more records become available.
ASSOCIATION WITH SOILS.
The nature of the soil plays an important part in determining the
weed-flora. The first idea was that certain weeds, as individual species,
would be characteristic of certain soils. This rarely proved to be the
case, but instead groups of species are characteristic. There is no hard
and fast line between the weeds occurring on different soils: with few ex-