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:—
| 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 | 4½ | 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 | 10½ ,, |
| 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 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.