The following pages give the results of observations on the dates at
which the commoner plants flowered at Brookthorpe, near Gloucester, as
well as the dates of a few other facts, such as the days in which the
songs of birds were first heard.
My observations began in April 1917, originating in the obvious lateness
of some of the vegetation. The record extends from 1st April to 21st
August, and contains only 160 observations, whereas in Blomefield's
_Naturalist's Calendar_, {231b} with which I have compared them, the
number of recorded facts is much greater. I may express my indebtedness
to the minutely accurate work of this author; I only wish that my small
contribution to his subject were more worthy of my guide.
What interest my observations may possess depends on the fact that the
spring of 1917 was exceptionally cold. For this statement I rely on the
weekly Weather Report of the Meteorological Office, in which for each
week of the year the deviation from the normal temperature is given for a
large number of stations in the British Islands. {232} I have taken as a
standard the temperature at Clifton, which seems to be the station
nearest to Gloucester.
Now, though the temperature has undoubtedly a great effect on the time of
flowering, it is by no means the only element in the problem. The first
plant on my list is _Ranunculus ficaria_, which I noted as flowering on
1st April, whereas in Blomefield the mean of seventeen yearly
observations is 28th February, the earliest date for this plant being
21st January, the latest 28th March. The extreme lateness of the
Celandine was doubtless due to the cold spring of 1917. But what are the
elements of the problem which fixed on this plant the general habit of
flowering early in the year?
In some cases we can see the advantages in early flowering. Thus the
average date on which the Hazel comes into bloom is 26th January, and
this, for a plant of which the pollen is distributed by the wind, may be
an advantage, since there are no leaves to obstruct the dispersal of the
pollen grains.
It may be answered that those Conifers which do not shed their leaves in
winter, _e.g._ the Yew or the Scotch Fir, are nevertheless
wind-fertilised. But this, though a point not to be forgotten, is no
argument against what has been said of the Hazel.
On the whole, however, we are excessively ignorant as to the biological
meaning of the dates at which plants flower. What advantage does the
orchis _Spiranthes_, well called _autumnalis_, gain from flowering in
August or September? Or again, what biological characters are there to
distinguish the plants flowering in June from those which do not show
themselves till July? It looks, to put the thing fancifully, as if a
parliament of plants had met and decided that some arrangement must be
made since the world would be inconveniently full if they all flowered at
once; or they may have believed that there were not enough insects to
fertilise the whole Flora, if all their services were needed in one
glorious month of crowded life. Therefore it was ruled that the months
should be portioned among the aspirants, some choosing May, others June
or July. But it must have been difficult to manage, and must have needed
an accurate knowledge of their own natural history. I must apologise for
this outbreak, and I will only add that this does seem to me an
interesting problem, namely, what are the elements in the struggle for
life which fix the dates on which plants habitually flower?
The most striking instance of the effect of the temperature is the
behaviour of arctic plants. {233} In Nova Zembla the summer consists of
two months, July and August, during which the mean temperature is about 5
degrees C. In these conditions, cases such as the following occur: at
Pitlekaj the last nine days of June showed a mean temperature of below 0
degrees C., while the average for the first nine days of July was between
+4 and +6 degrees, and on 10th July all the four species of Willow were
in full bloom, the dwarf Birch, _Sedum palustre_, Polygonum, Cassiope,
and Diapensia were in flower, and within a week the whole vegetation was
flowering. There was, in fact, a great rush or explosion of all sorts of
flowers as soon as the temperature rose: not that dropping fire which
begins with us with Mezereon in January and ends with Ivy in the autumn.
In the Arctic Regions temperature seems the absolute master, but in our
climate this is clearly not so. The best evidence of an inherent
tendency to flower on a certain date is that given by Askenasy {234} in
his observations on _Prunus avium_ (the Gean or wild Cherry). He
recorded the weight of 100 buds at regular intervals throughout the year,
and thus got the following results:--
_Grams_.
1st July 1 Period I.
1st August 2
1st September 3
1st October 4
1st November 4 Period II.
1st December 4
1st January 4
1st February 4.5 Period III.
1st March 6
2nd April 23
8th April 43
There are thus three periods: I., Formation; II., Rest; III.,
Development. So much for preliminaries; the really interesting point is
the reaction of the buds to forcing by artificially raising the
temperature. Thus branches put into a warm room at the end of October
showed absolutely no tendency to develop. In December, however, they
could be forced, and as time went on they proved to be more and more
amenable to the effect of a rise in temperature. In other words, the
invisible process of preparing for the spring was automatically
proceeding. The following figures give the number of days of forcing
needed at various dates to make cherry branches flower:--
14th December 27 days
10th January 18 ,,
2nd February 17 ,,
2nd March 12 ,,
11th March 10.5 ,,
23rd March 8 ,,
3rd April 5 ,,
My object in discussing this case is to show that the effect of
temperature on plant-development is not a simple problem. The most
picturesque association with what is known as the science of Phaenology
(_i.e._ the lore of the appearance of flowers) is its practical
connection with ancient agricultural maxims. Blomefield puts the thing
very clearly {235}: "The middle of March may be, in the long run, the
most suitable time for sowing various kinds of grain," but the husbandman
may easily go wrong in this or other operations if he sticks to a fixed
date. But if he knows that the conditions necessary for his purpose are
also necessary for the flowering of some familiar herb, he will be safer
in waiting for his guide to show itself than in going by dates. Wrongly
or rightly, this assumption has been commonly followed.
Stillingfleet quotes from Aristophanes that "the crane points out the
time of sowing" and the kite "when it is time to shear your sheep." An
old Swedish proverb tells us that "when you see the white wagtail you may
turn your sheep into the fields; and when you see the wheatear you may
sow your grain." I have come across an English proverb: "When the sloe
tree is as white as a sheet, you must sow your barley be it dry or wet."
Miss Jekyll in her book _Old West Surrey_, speaking of the wryneck,
quotes: "When we hears that, we very soon thinks about rining (barking)
the oaks."
There is something delightfully picturesque in the thought of man thus
helped and guided in some of his most vital operations by the proceedings
of the world of plants and animals, to whom that hard task-master Natural
Selection has taught so much.
I have gone through Blomefield's _Calendar_, recording for each species
the number of days between the earliest and latest known dates of
flowering. Thus the Mezereon did not flower earlier than 11th January or
later than 2nd February; this means that the date of flowering may, as
far as we know, vary to the extent of twenty-three days.
If we look at the recorded dates for all flowers appearing in February,
we find great irregularity. Thus _Daphne laureola_ has a range of
twenty-two days, whereas for _Vinca minor_ the figure is 114. The
average for February is 75.6, that for March is 55.6, for May 29.5, July
29.6. These figures suggest that the range of dates of flowering
diminishes as the temperature becomes less variable. But the variation
in summer temperature, though small relatively to the same factor in the
cold months, may nevertheless be sufficient to affect the flowering
habit. Yet there must be many factors in the problem of which we know
nothing. It is a curious little fact that the summer range should be
roughly one month.
Let us now consider my observations for 1917 as compared with
Blomefield's record of the mean date of flowering of the same species.
The most striking feature occurs at the beginning of April, when
Blomefield's observations are on the whole markedly earlier than my
record of corresponding facts. Of those noted by me as flowering in
April, one should have flowered in January, four in February, five in
March, six considerably earlier in April, and two slightly earlier in
that month.
In May Blomefield's dates are still mainly earlier than mine, in spite of
the fact that in this month the temperature was above the normal. In
June, on the whole (though with much variability), his dates do not
seriously differ from mine. In the first three weeks of June the
temperature was above the normal. In July, except at the beginning and
end of the month, my observations are clearly later in date than
Blomefield's, and during rather more than half of July the temperature
was below the normal. On the whole, and in spite of many doubtful
points, the difference between my results and Blomefield's seems to me to
be related to the curve of temperature, in an irregular manner it is
true, but sufficiently to be worthy of record. It has been said {237}
that Thoreau, the American recluse and naturalist, knew the look of the
country-side so intimately that had he been miraculously transferred to
an unknown time of year, he would have recognised the season "within a
day or two from the flowers at his feet." If this is true, either
American plants are much more businesslike than ours (which is as it
should be), or else Thoreau did not test his opinions too severely, and
this seems even more probable.