PERIODICITY IN ORGANIC LIFE.
55
period of scarcity is approaching. The only thing man has done, or
can do, apparently, to diminish the evil effects of the disease, has
been to cultivate those forms of the potato which have been proved
the best to resist attack.
Has weather had any effect ? Certainly it has to some extent;
heat and moisture always encourage the growth of fungi. But an
examination of the recorded temperatures of the disease years since
1848, will show that, as frequently as not, the bad years have been
cold as often as hot ones—in short, as far as we can see, meteoro-
logical effects have had little or no influence in promoting or
preventing the appearance of this disease.
Some years since, that condition of certain corn and grass seeds
known as Ergot was very prevalent in Essex. It was the period of
prosperity of the Ergot fungus.
Now we appear to be approaching the time of scarcity, and can
anyone say that man's proceedings favoured the abundance of the
fungus or that meteorological conditions reduced it ? The Ergot
has had its abundance in our county, and although now rare, it
certainly will appear again.
Those who live in Essex will sometimes notice, it may be
for some years in succession, how very abundant the wild oat
(Avena fatua) is, in wheat or other crops, over large districts of the
county.
Why is this ? Is it not an illustration of the same law, that
everything has its period of abundance and the contrary ? I may
be told that the wild oat is more common in wet years, but some
further explanation more than this is required, because I have
noticed them just as abundant during a succeeding dry year.
Parasitic plants, like fungi and disease germs, are good examples
of periodicity. Who has not noticed the extreme abundance for a
few years of the Orobanch of the clover (Orobanche minor) and then
its almost entire disappearance for a shorter or longer period.
Meteorological effects seem here also to exert no influence,
and, as far as I know, no explanation can be given for its abundance.
We can only say it is undergoing, from some cause, a period of
prosperity.
About 1844, there appeared in British waters a plant from
America (Anacharis alsinastrum) which could have been well spared.
It soon overran the whole kingdom and threatened to block up all
our more slowly flowing rivers and canals.