FLOWER FORMATION

The first essential phase in the life of a plant was the development of the embryo during which all of the essential organs of the vegetative plant were molded. A second crucial phase was germination, the transition from the seedling, which is dependent upon the reserve materials of the parent plant, to the autonomous, photosynthetically active young plant. This transition is safe-guarded by many control mechanisms. 

Few important new phenomena are encountered in the subsequent vegetative development. In the main those structures that were already present in the thriving seedling are further developed. We have considered in some detail a system of the kind that is developed particularly intensively during the vegetative phase of growth, namely, the vascular system. 

By induction of flowering we mean all those processes that cause the cells of the shoot meristem to switch from the course of development that they have hitherto pursued and now form the leaf organs of the flower instead of leaves. Once this pivotal event has been initiated, the switches are thrown so that the processes of differentiation leading to flowers or inflorescences foWo w (Fig. 230). The induction of flowering is of particular interest to the physiologist and the plant grower. This is because differentiation follows the induction of flowering almost automatically. Control of flower 

Now a few technical terms. During flower induction plants must be exposed to certain external conditions for a defined period of time. The time interval is known as the induction period and the efficacious external conditions as the inductive conditions. The natural, inductive, external conditions can, in many cases, be replaced by treatment with certain chemicals. External conditions under which plants remain in the vegetative state are called noninductive. 

In the following sections we shall consider in some detail the two most important inductive external conditions, defined conditions of temperature and light. First we shall collect facts together and then try to fit the individual pieces of data into an overall picture which is still necessarily hypothetical. 

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Hartsema, A.M. (1961). Influence of temperature on flower formation and flowering of bulbous and tuberous plants. Handbuch der Pflanzenphysiologie, 16, 123-67. 

The discovery of the effects of gibberellin on flower formation is closely linked 1 with older studies on the physiology of this process, particularly its regulation by certain external or environmental factors. 

Flower formation is usually obtained only after repeated applications of relatively substantial gibberellin doses. In lettuce, two treatments of 20 / g. each,... 

Most cold-dependent plants also possess a long-day requirement. The gibberellin treatment of these plants was usually conducted under long-day conditions, so that only the temperature-controlled block to flower formation was present. If the treatment was given under short-day conditions, only stem elongation was obtained it is evidently difficult to overcome the simultaneous presence of two blocks to flower formation. 

Can we say anything more precise about the function of gibberellins in flower formation Most plant biologists feel that flower formation is controlled by specific, hormone-like substances, so-called flower hormones. Attempts at ex-... 

The precise function of gibberellin in flower formation is by no means clear and may be indirect in nature. On the other hand, gibberellins are the first... 

Flower sex expression of cucumbers can be modified by treating seedlings with several growthregulating substances With the exception of gibberellin, chemicals tend to reduce the number, or suppress the development, of staminate flowers, and increase the number or accelerate the development of pistillate flowers Gibberellin, in contrast, increases the number of staminate flowers in monoecious cucumbers and results in the formation of staminate flowers on gynoecious cucumbers which otherwise would produce only pistillate flowers In promotive effects on staminate flower formation and vegetative extension, GA4 is approximately tenfold more active than GA3 ... 

Photoperiod enhances the promotive effect of gibberellin on staminate flower formation in the gynoecious cucumber not by increasing the number of nodes which... 

Table V Effect of Gibberellin and Photoperiod on Promotion of Staminate Flower Formation on Gynoecious Cucumber (MSU 713-5)...

The promotion of staminate flower formation in cucumbers by gibberellin is not surprising if one considers the effects of gibberellin as similar to those of a long photoperiod. Extension of the staminate stage in monoecious cucumbers by gibber-... 

Table VI. Comparative Activities of Gibberellin A3 and Butyl Cellosolve Ester of Gibberellin A3 on Promotion of StamSnate Flower Formation on Gynoecious...

Lang, A. (1957). Effect of gibberellin upon flower formation. Proc. Natl. Acad. Sci. USA 43,709-717. 

Zeevart, J.A.D. (1958). Flower formation as studied by grafting. Mededelingen Landbouwhogesschool, Wageningen, 58, 1-88. 

CORYMB.—A collective flower formation, in which the flower stalks are of differing lengths, but each rises to the same level, outer flowers being the first to open. 

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