THE FLORAL ORGANS. 



81 



393. Convolute, when each leaf wholly involves all that are within 

 it, as do the petals of Magnolia ; and vexillary, when one piece larger 

 than tbe rest is folded over them, as in the pea (251). 



394. Plicate or folded aestivation occurs in tubular or monopetalous 

 lowers, and has many varieties, of which the most remarkable is the 

 supervolute, where the projecting folds all turn obliquely in the same 

 direction, as in morning-glory, thorn-apple (Datura). 



Diagrams of flowers (as seen by cross-sections). 253, Jeffersonia diphylla : o, ovary ; s, sta- 

 mens ; d, inner row of petals, aestivation triquetrous ; I, outer row of petals, {estivation contorted ; 

 e, sepals, aestivation quincuntial. 254, Lily. 255, Strawberry. 256, Mustard. The pupil will 

 designate the modes of aestivation. 



Tho aestivation of the sepals often differs from that of tho petals in tho same 

 flower. Thus, in the pink the sepals are imbricated and the petals contorted. 



395. The position* on the parts op the flower, with respect to the main 

 axis and the bract whence it arises, is often important in description. That part 

 which is adjacent to the axis is the posterior or upper, while that which looks to- 

 ward the bract is tho anterior or lower part. 



THE FLORAL ORGANS. 



396. Technical definition of the flower. Tho flower is an as- 

 semblage of leaves more delicately and variously formed, borne at the 

 upper nodes of the axis where the internodes are undeveloped. This 

 portion of the axis is called 



397. The receptacle or torus. It is the axis of the flower situ- 

 ated at the summit of the flower-stalk. Its form above is commonly 

 that of a flattened or somewhat conical disk, the center of which cor- 

 responds with the apex of the axis. 



398. The flower mat consist of the following members : 1, the 

 floral envelops ; 2, the essential floral organs. 



399. The floral envelops consist of one or more circles or whorls 

 of leaves surrounding the essential organs. The outer of these whorls 

 is called the calyx and the other, if there be any, the corolla. The 

 calyx may, therefore, exist without the corolla, but the corolla can not 

 exist without 



400. The calyx. This is a Greek word signifying a cup. It is ap- 

 pliod to the external envelop of the flower, consisting of a whorl of 



