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A Procession Of Flowers |
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Title: A Procession Of Flowers Author: Francis Darwin [More Titles by Darwin] {231a}
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° 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° C., while the average for the first nine days of July was between +4° and +6°, 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:—
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:—
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 Phænology (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. NOTES. * This column gives Blomefield’s mean dates. + S is the date on which the song was first heard. L is the date of leafing. N that of nesting. The other entries are the dates of flowering.
FOOTNOTES
{231b} A Naturalist’s Calendar kept at Swaffham Bulbeck, Cambridgeshire. By Leonard Blomefield (formerly Jenyns). Edited by Francis Darwin. Cambridge: at the University Press, 1903. {232} I am also indebted to Mr Embrey for his kind help in this matter. {233} Kjellman, in Nordenskiold’s Studien und Forschungen, 1885, pp. 449, 467. {234} Botan: Zeitung, 1877. {235} A Naturalist’s Calendar, p. xii. {237} The Times Literary Supplement, 12th January 1917, p. 326. [The end] GO TO TOP OF SCREEN |