AIS Bulletin (1923) pp. 17-19
Bud Development in Iris
J. Marion Shull
Doubtless many people are familiar with the fact that bulbs such as Narcissus, Hyacinths and Tulips have their flower buds already formed within the bulb, waiting only the right conditions of temperature and moisture to bring them quickly into bloom, but it may not be so well known that the Iris also takes thought of the morrow and provides for the next season's bloom before it goes to rest. A realization of this fact may help to explain why, in a spring like that of 1921, so many of them failed to bloom or came stunted or crippled, and may also suggest some difference in our treatment or the Iris in after-summer if we would have abundant bloom the following spring.
A few years ago the writer began a study of immature Iris buds, first examining rhizomes in early spring before growth had visibly started. These observations, and the illustrations presented herewith, are all from an old variety once catalogued as Virgile but now apparently superseded and dropped from most lists. These buds taken in the spring were found in such a stage of advancement that further observation was postponed until the following autumn, but they brought out one very striking fact, that the Iris flower develops its different parts unequally and at different times. The stamens, finally the least of the various organs, are here the most prominent, while falls and standards, destined at length to be the greatest, are here the merest teeth surrounding the base of the stamens.
Buds were taken in November and Figure 1, a, is from a camera-lucida drawing showing the entire inflorescence, with the last pair of sheathing leaves, all compressed till it is little more than one-tenth of an inch in height. Removing the two leaves the entire flower stem is shown at b on a larger scale, and at c one of the most backward buds of this inflorescence, a bud scarcely more than one fiftieth of an inch in diameter, is shown on a still larger scale. So much for the development of the bloom at the close of autumn, but it should be added that such buds have been found on specimens of Mrs. H. Darwin as early as in September, and it is not improbable that early varieties like Florentina or the common blue may have begun setting buds even in July or August, tho no data on these is at hand.
|Fig. 1.—Iris inflorescence as developed at close of growing season (November). a., with last two sheathing leaves. b., flower stem with leaves removed. c., the backward bud shown at left in b., still more highly magnified.|
Figure 2 is a flower stem taken April 4, 1919, and shows but little advancement over that taken in November. Its most advanced flower is drawn separately. Here, the stamens are well formed, the whole flower can hardly be said to resemble or even suggest the Fleur-de-lis. The style-branch, which at maturity arches over and is many times larger than the stamen, is shown at a, greatly magnified, for it is still a mere lip about one hundredth of an inch across and projecting about half that distance into the throat of the flower.
|Fig. 2. —Iris inflorescence taken April 4. 1919. a., style-branch from flower illustrated, highly magnified.|
Figure 3 is a complete flower stem taken one week later and drawn to the same scale, and indicates considerable growth. The style-branch begins to show two rounded lobes that later develop into the handsome toothed or fringed crests of the mature flower, while the triangular portion at tip represents the stigmatic lip where eventually the pollen must fall if seed is to be produced.
|Fig. 3.—Iris inflorescence taken April 11, 1919, one week later than in Fig. 2, and drawn to same scale. a., the most advanced flower at this stage, and b., the style-branch from same flower, showing beginning of crests and stigmatic lip. c., a flower bud taken one week later than a.|
Perhaps all this seems only calculated to satisfy one's curiosity, but it may not be devoid of practical bearing if we will bear in mind that August, September and October are the critical months so far as bud formation is concerned; that the growing conditions of this period will determine whether our clumps will be graced with many or few flower stems the following spring. It would certainly indicate that the best time for resetting is very shortly after blooming, or, failing that, not until after the buds have had time to form, with perhaps just time left for roots to take good hold of the soil for the winter.