362. THE STRUCTURE OF ECONOMIC PLANTS 



mordia of the sepals, zonal growth takes place at their bases, and 

 the continued growth of this zone and the tips results in a cylin- 

 drical tube with five acuminate lobes at its outer margin. This 

 forms a protective covering over the growing point and other 

 floral parts. (Fig. 183, £.) 



As the growth of the calyx proceeds, the five petal primordia 

 arise from the growing point alternate with the sepal lobes, the 

 two primordia which form the keel of the corolla being formed 

 first, followed by those of the two lateral wings and the standard. 

 The primordia of the keel are at first distinct structures; as growth 

 proceeds, their adjacent edges become more or less completely 

 united along the abaxial margin of the keel; and, at maturity, 

 they form a hood-like structure which partially surrounds the 

 stamens and carpel. (Fig. 184, F, G, H.^ Although the petal 

 primordia are the second cycle of organs to be differentiated, they 

 grow very slowly at first, remaining small during the early develop- 

 ment of the stamens. 



In the mature flower, the stamens are diadelphous and appear to 

 be arranged in one whorl in which nine stamens are "united" by 

 their filaments, while the tenth, adaxial stamen is free. This is 

 due to the perigynous development of the flower, in which there is 

 non-divergence of the basal portions of the stamen primordia, and 

 the early ontogeny indicates that the flower is actually pentacyclic 

 with two whorls of stamens, each consisting of five primordia. 

 (Fig. 184, H.) The development of two cycles of stamens as noted 

 above has also been reported for Trifolium, Melilotus, Phaseolus 

 and other legumes. 



As a result of the growth of the tissue basal to the whorls of 

 stamen primordia, the bi-cyclic character of the androecium is soon 

 obscured, and nine of the stamens develop with a common basal 

 collar of non-diverged tissue, leaving the tenth stamen free. (Fig. 

 184, G.) This stamen is a member of the inner whorl, and the 

 elongation of its free filament maintains the anther at the same 

 level as those of the other members of the whorl. The maturation 

 of the filaments and anthers of the outer whorl is more rapid than 

 that of the inner, and the outer stamens produce mature pollen 

 grains earlier. (Fig. 183, £.) 



Immediately following the differentiation of the inner cycle of 

 stamen primordia, the carpel primordium is differentiated, and 

 grows so rapidly that it soon becomes larger than any of the other 



