1090 A TEXTBOOK OF THEORETICAL BOTANY 



Flowers are usually borne on lateral axes arising from the axil of a foliar 

 organ called the bract, which is occasionally indistinguishable from a 

 foliage leaf, but is usually different in size and often also in shape. It is not 

 infrequently reduced to a scale-leaf, corresponding to the base of a foliage 

 leaf, and it may rarely be entirely absent. The posture of the flower in 

 relation to the axis and the bract varies considerably but is generally fairly 

 constant throughout wide related groups such as Orders. This posture of 

 the flower is defined by the position of the outermost (lowest) sepal, 

 particularly in relation to the bracteoles, which arise on the pedicel in close 

 relation to the flower. The bracteoles seem to have been primitively the 

 bracts of further flowers, and in some cases axillary flowers arise from them, 

 either as part of the normal branching of a cymose inflorescence or in other 

 cases as occasional abnormalities, but in the majority of cases these lateral 

 flowers are aborted and in many instances the bracteoles are so closely 

 integrated with the flower which terminates the pedicel that it is impossible 

 to say whether they ever functioned as bracts. The presence or absence of 

 bracteoles is often very variable, even in closely related species, though whole 

 genera, e.g., in the Leguminosae, may be without them. 



The lateral vegetative branches of Dicotyledons normally begin with 

 two laterally placed prophylls and those of Monocotyledons with a single 

 prophyll, posterior to the branch, that is, between its base and the main 

 axis. These may be considered as the " cotyledons " of the new shoot. 

 A similar rule holds for the flowers; Dicotyledons having two lateral 

 bracteoles and Monocotyledons one in the posterior position. If bracte- 

 oles are absent the first sepal may either occupy the position where 

 the first bracteole would have stood or, more frequently, the position 

 which it would otherwise have occupied if the bracteoles had been 

 present. For example, in Cruciferae the lowest pair of sepals lie in the 

 plane of the axis, as if the two abortive lateral bracteoles were actually 

 present. 



Displacements of the bracteoles from their true morphological position 

 are not uncommon. Both the bracteoles of a Dicotyledon may be displaced, 

 anteriorly or posteriorly, and in other cases one of the two may be displaced 

 to right or left respectively and placed at the beginning of the sequence 

 of sepals so that in efl'ect it becomes a sepal. This close relationship of 

 bracteoles and sepals is shown even more clearly in those Dicotyledons 

 which have multiple bracteoles. In these flowers a succession of bracteoles, 

 usually paired, are closely set below the flower to form an involucre, good 

 examples being Camellia and Chimonanthus (Fig. 1058). In many such cases 

 it is impossible to say exactly where the involucre ends and the calyx begins. 

 One cannot avoid the conclusion that the calyx is a progressive development 

 from bracteoles rather than a retrogressive corolla, and the same applies to 

 those cases in which the whole perianth is simple and consists only of 

 sepaloid members. Such an apparent continuity between bracteoles and 

 sepals is not however universal, for the former may sometimes be indepen- 

 dently developed into an enlarged involucre which completely envelops 



