Flower Bud Differentiation and Development in Cattleya labiata Lindl.
Gavino Rotor, Jr. and L. H. MacDaniels

Cattleyas are the most important commercial group of orchids.. Very little is known about the bud initiation and development in this group. This is surprizing in view of the fundamental nature of this problem and the possible bearing it may have on current cultural practices. The presnt study was undertaken to provide information on this important problem.

Brown (1831), Cruger (1865), Darwin (1899), and Lindley(1830) were among the earlier workers who described the flowers of various orchid species. More recently, Edwards (1941) and Swamy (1948) studied the vascular anatomy of orchid flowers. These investigations were concerned primarily with comparative morphology and phylogeny. From the horticultural standpoint Johnson (1943) reported on flower initiation and development in the orchid Cattleya pinole. In this work it was concluded that flower bud formation was definitely related to the length of the vegetative shoot or developing pseudobulb. Aside from this study, no other information on the time of bud initiation and development in Cattleyas was found.

Cattleya labiata Lindl. flowers normally from late summer until the early part of November. It is regarded as a fall flowering species. The pseudobulbs of the current year usually mature by the end of June under usual greenhouse conditions in Ithaca, N. Y. The inforescnce develops from the apex of the new pseudobulb. This part of the pseudobulb was therefore used in studying bud initiation and bud development. The tips of the pseudobulbs were collected and examined at regular intervals of 2 weeks starting May 28, 1949, to determine the normal date of bud initiation. After July 9, 1949, samples in the bud initiation stage could not be found. By June 25, all the plants had leads with fully-formed pseudobulbs. Ninety percent of these were well-matured with fully expanded leaves. The rest had partly unfolded leaves which were about two-thirds out of the enveloping sheaths.

In determining bud initiation, Johnson (1943) took samples from vegetative shoots of differing lengths. This is the only work reported so far on bud initation in Cattleyas and the method used was followed at the start of the present study. When it became evident, however, that there was no relation between shoot length and bud initation the method was discontinued. All the materials presented in this paper were collected from mature or fully-formed pseudobulbs with well-developed, expanded leaves.

The samples were fixed in formalin-aceto-alcohol, embedded in paraffin by the tertiary-butyl alcohol method, sectioned at 8 - 10 microns, and stained with iron haematoxylin and safranin according to Johanson's schedule (Johansen, 1940).

Before flower bud differentiation, sections of the meristem at the apex of the pseudobulb show a small rounded tip surrounded by a few bracts (Figs. 1, 7A). Flower bud initiation is first apparent by an increase in meristematic activity and elongation of the apical meristem. Bracts and somewhat flattened protuberances are differentiated alternately on both sides of the inflorescence axis. These flattened protuberances are the flower primordia (Fig. 7B, C). In the next stage an invagination develops in the flattened primordium resulting in the formation of a meristematic disk with a slightly depressed center (Figs. 3, 4, and Fig. 7C, D). Upon this disk two protuberances develop (Figs. 4, 7D). The upper lobe next the main axis later forms the two ventral sepals, the labellum and the two lateral petals; the lower lobe forms the dorsal sepal and the column. The ventral sepals appear a little earlier than the single dorsal sepal (Figs. 4, 7D).

Closely following the differentiation of the sepals is the development of the labellum, the two lateral petals, and the column. All these structures appear to be differentiated simultaneously although their growth rates are different. Judging from the serial sections the two ventral sepals develop most rapidly followed by the labellum, the two lateral petals, the dorsal sepal and lastly the column. At the base of the column an invagination appears resulting in the formation of a head-like structure (Figs. 7E, F). At this time the sepals are short and at some distance from each other so that the inner whorls comprising the lateral petals, labellum, and column are partially exposed and the top of the bud remains open. At this stage of bud development the whole inflorescence inside the flower sheath first becomes vaguely discernable as a short conical structure about 3mm. long.

In subsequent development the sepals expand laterally and elongate repidly, gradually enclosing the inner parts which slowly increase in size but remain comaratively short. The column grows slowly annd a groove develops on its ventral surface throughout its whole length. This marks the initiation of the anthers. In Figure 7I, the longitudinal section of a bud 1.5-cm long, shows all the parts differentiated. Further development consists of the expansion and elaboration of the differentiated parts. The most conspicuous changes during this period of development are observed in the column and ovary. Both structures increase rapidly in length to several times their width. (Fig. 8C, D, E, F, G, H, I, J, K). At anthesis the ovary is about as long as he sepals and petals.

From the early stages of develoment to emergence of the inflorescence from the flower sheath, the column faces the inflorescence axis (ventral position) and the labellum, which is opposite and slightly above teh column, faces outwards. Further development results in the inversion of the flower bud by the twisting of the ovary so that at the time of anthesis the labellum is below the column while the latter is turned away from the axis. This phenomenon in orchids has been observed in many other species and discussed at some length by various investigators (Darwin 1899; Ames, 1938). Edwards (1941) concluded from a study of the vascular bundles that the inversion of the flower is an actual twisting and seems to be a tropistic response.

The inflorescence of Cattleya labiata, Lindl. is indeterminate and under favorable conditions has been observed to produce six flowers. Orchid growers generally obtain only two to three and sometimes four flowers on one inflorescence. It is during the period of bud differentiation and development that environmental facors such as water and temperature are most critical from the standpoint of flower production. At this time these factors may greatly influence the continued growth of the apical meristem while it is differentiating the flower primordia and hence affect the number of flowers. Arrested development annd degeneration of the terminal buds was observed in some of the sections examined. This condition may occur at any time during this period if conditions are unfavorable.

Normal bud initation with Cattleya labiata, Lindl. was observed in 1949, under Ithaca conditions, in late June. Before this time samples taken from mature and from young, developing pseudobulbs did not show any sign of bud initation. After June 25, bud differentiation was found in various stages in samples taken from fully formed, though not necessarily mature, pseudobulbs of the current year's growth. No relation could be established between length of the pseudobulb and bud initiation. The statement that there is such a relation (Johnson, 1943) might possibly be explained by the collection of samples at the time when Cattleya pinole normally forms its buds. The time of the year the samples were taken was not stated. The present study shows that no matter what the length of the vegetative shoot, provided it was fully formed, buds will be differentiated when the time for bud initiation comes. It has been observed and it is a well-known fact among orchid growers that mature pseudobulbs without flower sheaths are capable of forming flower buds long after the leaf blade is fully expanded as indicated by the appearence of buds on such shoots; also, large buds may emerge from developing pseudobulbs even when the leaf is not yet fully unfolded. Johnson (1943) found "eveidence of some latitude in time of initiation. Primordia in some shoots 17 cms. long were more advanced in development than other primordia form mature shoots."

SUMMARY

Flower bud initiation in Cattleya labiata, Lindl. occurred in the last week of June in 1949 at Ithaca, N. Y., under usual greenhouse conditions. The inflorescence was found to be indeterminate and the flowers formed in two ranks in the axils of the bracts. The sepals were the first floral parts to be differentiated. The ventral sepals developed most rapidly followed by the labellum, the two lateral petals, the dorsal sepal, and lastly, the column. The two ventral sepals, the labellum, the two lateral petals and the two stigmas are all derived from the upper lobe of the invaginated primordium while the lower lobe gives rise to the dorsal sepal, the column and all the structures present on the column, e.g., anthers, anthercap, and the rostellum. The invagination of teh floral primordium later developes into the cavity of the ovary. At the time when the whole inflorescence inside the flower sheath becomes vaguely discernible as a short conical structure by holding the sheath to the light, the buds ar already well developed with all the parts differentiated. At this stage the inflorescence is about 3 mm. long. The buds of Cattleya labiata, Lindl. exhibit the phenomenon of inversion during an interval of 3 days between complete emergence from the sheath and opening. The most critical period as far as flower production is concerned starts at the time the flower buds are initiated and extends to the stage when they are fully differentiated. No relation was established between the length of the pseudobulb or the stage of leaf development and flower bud initiation.

LITERATURE CITED

Ames, O. 1938. Harvard Univ. Bot. Mus. Leaflet 6:145-183.

Brown, R. 1831. Observations on the organs and mode of fecundation of Orchideae and Asclepiadeae. Trans. Linn. Soc. London 16:685-745.

Cruger, H. 1865. A few notes on the fecundation of orchids and their morphology. Jour. Linn. Bot. London 8:127-235.

Darwin, C. 1899 The various contrivances by which orchids are fertilized by insects. 2nd ed. London.

Edwards, A. T. 1941 Anatomy of the orchid flower with reference to inversion of flower in some genera. Unpublished thesis. Cornell University.

Johansen, D. A. 1940 Plant microtechnique. McGraw-Hill Book Co. New York.

Johnson, E. 1943. A study of flower initiation and development in orchid Cattleya pinole. Amer. Orch. Soc. Bull. 11:422-425.

Lindley, J. 1830. The genera and species of orchidaceous plants. London.

Swamy, B. G. G. 1948. Vascular anatomy of orchid flowers. Harvard Univ. Bot. Mus. Laeflet 13:61-95.