Bees Don’t Care – Hybridizers Should

Whenever dahlia people discuss open-centered type dahlias, especially during the judging process, we often hear reference to so many rows of stamens, not enough or too many stamens, etc. The Original Judging Manual stated…”in the open-centered types the disc in perfect show condition should present no more than two rows of pollen bearing stamens…” That wording has been carried through to the latest revision. One drawing in the current Judging Manual labels those appendages “stamens”.

This seems to have led to wide spread misunderstanding of the botany of dahlias and the mechanics of cross pollenization. Several hybridizers, when discussing hand pollination, as well as several articles in the ADS Bulletin have reported using a brush to collect pollen from one parent to brush on the other, or emasculation of one or both parents and rubbing one on the other. Considering these two operations, which are apparently the two most common methods used, one gets the feeling that those yellow appendages around the periphery of the disc of a dahlia are considered by many growers to be those “pollen bearing stamens”. Not so!

Either the brush transfer or emasculation method will work satisfactorily if properly and timely carried out. It is hoped this article will provide a better under standing of just what goes on during this procedure and possibly lead to more effective fertilization.

First, it should be stated that dahlias have only one row of stamens (in each corolla tube – editor). They are extremely difficult to see without a high power magnifying glass or microscope and seldom if ever, protrude above the corolla tube which encloses them. Since the similar elements on large ray florets are much larger and not enclosed in a corolla tube, they can sometimes be seen with the naked eye.

The figure on the following page are not to scale nor perspective but are believed sufficiently descriptive to clearly explain the relation and operation of the reproductive organs of the disk florets of all dahlias

Glossary of terms

These terns are not necessarily true for flowers in general, nor even the ray florets of dahlias. Some articles have reported that seed is produced in the ray florets; others have made statements to the effect that only the disk florets are complete and fertile. The broad definitions applicable to all flowers in general have been modified to specifically apply to the open-centered type of dahlias, and the descriptions to follow. Each definition is keyed to the DRAWINGS. You may wish to open a new window to keep the drawing available while reading the text. It could also be printed.

a. ovulary – an expanded and more or less hollow portion of the pistil containing the ovule, which when properly fertilized develops into the seed.

b. corolla tube – a cylindrical tube of five membranous ligules fused together and enclosing the pistil and stamens. This tube is elongated and colored in the anemone types.

c – stalk a tubular extension of the corolla tube which joins that element in the ovulary, and through which the style passes.

d. – stamens the so called male elements of the reproductive system that produces the pollen by which the stigma is fertilized. They are coherent due to a 5-valved membranous covering thereby forming a circle inside the corolla tube. This tube in turn encloses the stigma and style.

e. – anthers the expanded tip of the stamens which produce the pollen.

f. – filament a threadlike chain of cells, acting as the stalk of a stamen

g. – stigma the bi-lobed tip of the pistil which expands to receive the pollen .They are covered with very small, sticky hairs.

h. – style the part of the pistil between the stigma and the ovulary.

i. receptacle the part of a modified stem that bears the flower parts and to which all ovularies are firmly attached.

A. depicts a disk floret in the immature stage. It consists of three basic parts and contains all the reproductive system elements in a rudimentary form.

B. depicts the floret at the beginning of dehiscence. To start this process, the growth of the style causes the stamens to force their way upward, spreading the corolla tube and allowing the anthers to become exposed. (These anthers seldom project above the corolla tube, making them extremely difficult to see without the aid of a strong magnifying glass.) After only a few hours the stamens begin receding and the stigma is liberated through the circle of stamens and the summit of the corolla tube. Since the stigma is surrounded by the coherent tube of stamens, it emerges covered with abundant pollen on the outer surfaces. (This action which results in pollination in many genera is quite ineffective in dahlias because of the self-incompatibility of the genus.) Pollination then must come from the out-crossing with another variety. This condition is depicted in

C. where the stamens are withdrawn and the stigma exposed well above the expanded corolla tube. If the withdrawn stamens are now examined they will be found to be brown with a diminished covering of pollen.

The smallness of the disk florets and early dehiscence make emasculation impracticable.

Thus the outer rows of pollen bearing elements are not the stamens as would normally be expected, and identified as above but in fact the stigma covered with pollen from the stamens. (If this phenomenon did not take placed hand pollination would be virtually impossible since the pollinizing agent would have to gather the pollen from inside the corolla tube). Fortunately the inside surfaces of the stigma are relatively free of pollen allowing the hybridizer whether a grower with a brush, or more probably the bee, ant, hummingbird or other agent to effect pollination. When the agent supplies pollen from another plant it also gathers some of the pollen for transfer to a third plant.

The stamens with their pollen covered anthers are extended for only a few hours. The stigmas are receptive a few hours after dehiscence and pollination takes place only at that time. The period of receptivity is also reported to be quite short. Since only a limited number of viable stigmas are present at one time pollination must be attempted several times during the day and over a period of two to four days.

Each disk contains from 110 to 150 corolla tubes, which if properly fertilized are capable of producing seeds. In nature seed development is more apt to be in the 25 to 50 range, Frequent pollination should increase the number of viable seed produced…

This is just one of the many interesting and unique aspects of dahlias, that make the flower so completely fascinating.

– Gerry Weland
extracted from the ADS Bulletin Sept.1994