Development of the flower and macrogametophyte of Allium cepa

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In central California mother bulbs of the onion (Allium cepa L.) used for seed production are usually set in the field during late November and December. Subsequently, a number of leaves are formed at each of the several growing points before the inflorescence axis is differentiated fig. 1, A). The method of leaf development has been adequately described by Hoffman.⁽²⁾4 Briefly, the leaves are two-ranked, the blade of each new leaf arising at an angle of 180° from that of the next older. That side of the apical meristem opposite the preceding blade is the first to differentiate; and as this region develops an upward growth of tissue soon completely encircles the growing point of the stem, differentiating the new leaf.

In Maryland, Jones and Boswell⁽⁴⁾ found that the primordium of the inflorescence axis differentiated in March when mature bulbs were planted in the field in October. In California, bulbs planted in December had floral axes differentiated in February.⁽³⁾ The first stages in the development of the primordium of the leaf and that of the inflorescence axis appear to be very similar. In the latter the single involucral bract is first evident at a point opposite the youngest leaf blade. At the first appearance of the bract, one cannot tell whether the new primordium is that of a leaf or of a bract.