BEES AND FLOWER-FERTILIZATION.

New Zealand Journal of Agriculture, Volume XVIII, Issue 4, 21 April 1919, Page 203

BEES AND FLOWER-FERTILIZATION.

THE CASE OF BEANS AND PEAS.

By

W. H. TAYLOR, Horticulturist.

The following notes are written with the idea of elucidating certain questions which have been under discussion for some time past, such as, “What perforates the-flowers of broad beans?” “Why do runnerbeans fail to set beans on the first flowers ?” “ Do bees cross-fertilize peas ?” The authority used by the writer in dealing with these questions is Darwin, in his book “Cross and Self Fertilization of Plants.” > ■

Regarding holes in the corolla of flowers, Darwin states that in England these are always made by humble-bees, but that hive-bees invariably avail themselves of the holes and rarely visit in the proper manner flowers that have been perforated by humble-bees. By the possession of powerful mandibles humble-bees are .particularly well equipped for making the holes ; their object is to obtain the nectar quicker than by entering the flowers, thus enabling them to visit a larger number of flowers. My own observations convince me that there is sometimes another reason. I have taken advantage of every opportunity that has offered this summer to watch the movements of humble-bees on plants in my garden. The first noticed were small black specimens, probably Bombus ruder atus, a considerable number of which' were at work on antirrhinums. The bees without exception alighted on the lower limb of a flower, and by their weight and movements depressed and opened it ; they then entered the mouth of the flower, forcing their way right in so as to reach the nectar. Later on the black bees disappeared and greater numbers of the large banded bee, Bombus terrestris, appeared. Not one of these visited the mouth of a flower, but gnawed holes in the corolla just above the nectary. In the course of a few days it was only by close search that flowers could be found without a hole in the corolla ; usually there were two holes, one on each side of the rib that runs down the lower limb of the corolla. In this case I conclude that the insects knew they could not reach. the nectar in the proper manner. They undoubtedly were too large to enter a flower.

RUNNER-BEANS.

Darwin states that the flower of runner-beans (Phaseolus multiflorus') is entirely self-fertilizing, and yet though such is the case it is quite incapable of fertilization without the aid of insects, the fertil-izing-organs being in a spirally wound keel from which they must be released to effect pollination. Bees visit the flowers continually. They alight on the left wing-petal, as they can best suck the nectar from this side. Their weight and movements depress the petal; this causes the stigma to protrude from the spirally wound keel, and a brush of hairs pushes the pollen before it. The pollen adheres to the head or proboscis of the bee which is at work, and is thus placed on the stigma

of the same flower or is carried to another flower. That this plant is self-fertile was proved by Darwin and others, who found that when plants were covered so as to exclude insects only very rarely was a bean formed, yet by moving the wing-petals with a pin fertilization took place and beans formed.

It will thus be seen that beans cannot be formed unless the flowers are visited by bees or some other insect sufficiently powerful to depress the wing-petals. The bees are, of course, quite unaware of the presence of the flowers until they see them. Bees do not detect flowers until a. considerable number have expanded, presumably because they are not sufficiently, conspicuous to attract their attention. The bees may not observe the first flowers on runner-beans, as they are near the ground and to some extent concealed by the'foliage. But they soon detect and visit them when many flowers are out, and at once beans are formed. This clearly explains why the first flowers fail to set beans. If bees perforated the corollas instead of visiting the flowers in the proper manner no beans would be formed, because the necessary movement of-the wing-petals would not occur.

BROAD BEANS

Although not quite sure on this point, I believe the agency of bees is necessary to secure fertilization of broad beans (Faba vulgaris).- If that is the case the reason why the perforated flowers fail to set beans is explained. It is the first flowers usually that are found perforated, and the question arises, Why is it ? The explanation is quite simple. I have previously mentioned my observations regarding antirrhinums and how they were constantly visited by humble-bees. About the middle of March in my garden a plant known as “ blue spiraea ” (Caryopteris mastocanthus) began to open flowers. The bees soon 1 found it, and after a day or two had abandoned the antirrhinums and concentrated their attention on the caryopteris. I counted as many . as twenty-four large banded humble-bees on the plant at one time. Since that time, so far as I have seen, not a single bee has visited the antirrhinums, which is not strange, as bees always confine their attention to one species while they can, and evidently the nectar of the caryopteris (natural order Verbenaceae) is preferred to that of the • antirrhinum (natural order Scrophulariaceae).

The first flowers on an early crop of broad beans open at a time when flowers of any kind are scarce, and the humble-bees make use of them. Later on, when more flowers of the beans have expanded, there are also many other flowers out, and the bees probably abandon the beans in favour of something more to their liking. So that an observer may watch in vain for more visitations by humble-bees on beans.

DWARF BEANS.

The flowers of dwarf - beans ■ (Phaseolus vulgaris) are entirely selffertile without the aid of insects, as is abundantly proved by their being grown in greenhouses where bees are not present. Yet they are capable of cross-fertilization by bees, and it undoubtedly occurs. The extent of cross-fertilization is not, however, . great, and different varieties grown in close proximity will remain’fairly pure, but not entirely so. . . ■ ■ ■

GARDEN PEAS.

The flowers of garden peas (Pisum sativum) are entirely self-fertile, and usually behave as though cleistogamic (uncrossable), which they certainly are not. Fertilization is effected before the flowers open, and. this precludes the possibility of crossing by wind-borne pollen. Notwithstanding these statements —and there is. abundant proof that they are true — flowers are obviously adapted to cross-fertilization. Darwin observes that it is remarkable that they are not often crossed, yet- it only very rarely happens. He mentions cases of . varieties that have remained pure for sixty , years, though each year several varieties were grown together. Hive-bees can have no effect on the fertilization of these flowers, as they are not heavy enough to open them, consequently they cannot gather pollen from them except from old and already fertilized flowers, which they sometimes do, but not to any great extent. For my own part, I have frequently watched my garden peas this season and have not seen a bee on them. Darwin states that he had peas under observation for thirty years, and only thrice during that period did he see bees of the proper kind at work. These were Bombus muscarum, a humble-bee. These, he is sure, must have crossed some flowers. • ■

SWEET-PEAS.

The flowers of the sweet-pea (Lathyrus odoratus) are entirely selffertile, and can rarely be crossed by insects and never by wind-borne pollen. I have examined a large number of flowers and proved that fertilization takes place long before the flowers open. However young a flower may be a pod will be found in it, and the smallest pod I could divide with a sharp knife exhibited peas quite distinctly.

BEES AND SEED-GROWING.

It has previously been stated that bees confine their attention as long as possible to one species. It is also known that when gathering pollen bees will not wander far if they can get their load nearby. The knowledge of these facts, and, further, that different orders of plants will not cross, guides seed-growers in planning their plots. All the different species of brassica, which include cabbages, cauliflowers, &c., will cross each other. The seed-grower plans his field so that no two varieties of the same species are close to each other. But this would not ensure safety from crossing unless a considerable number of plants of each variety were grown, for bees will fly quite a considerable distance in search of flowers of a species. Safety is found in growing a good block of each variety. The bees then load up from the one block and the risks of crossing are very small.

aterproofing Concrete. — Concrete can be made fairly waterproof by mixing in approximately 8 per cent, of clay , in powdered form. The strength of the concrete is not materially 'lessened, though its setting is likely to be lengthened considerably. - The result of a great many experiments, however, has been to show that good workmanship, a well-graded aggregate, and plenty of cement is the best way to render concrete waterproof. <C •