ORGANS OF REPRODUCTION. 213 



Another modification of imbricate aestivation occurs in the 



flower-buds of the Pea and other allied plants, where the superior 



petal 4, which is generally the largest, and called the vexillum, 



is folded over the others which are arranged face 



to face {fig. 435). This form of aestivation is 



commonly termed vcxiUary. 



It frequently happens that the calyx and corolla 

 exhibit different forms of aestivation. Thus, in 

 Guazuma idmij'olia the calyx is valvate, and the 

 corolla indiiplicate. In Malvaceous plants the 

 calyx is valvate or redujjlicate, and the corolla 

 contorted. In these two examples the different 

 forms of aestivation, as exhibited by the two 

 Fig. 435. Dia- floral envelopes, may be considered to belong to 

 frate ve^niary the same class of aestivation, i.e. the circtdar ; 

 ffistivntion. 1 'but instances also frequently occur where the 

 ai« o/w^ings,^3 forms in the cal}-x and corolla are different, and 

 aiid 5 the ca- belong to both classes. Tims, in the Bindweed 

 the' vexillum. {Calystegia), and other Convolvulaceae, the calyx 

 is quincuncial or imbricated, and the corolla con- 

 torted. A similar arrangement occurs in the Corn Cockle 

 {Agrostcrnma Githago), in the St. John's Wort {Hyjpericum), in 

 the Geranium, and in many other plants. 



The forms of aestivation above described are always constant 

 in the same individual, and frequently throughout entire genera, 

 and even natural orders ; henne they are of great importance in 

 systematic botany. For a similar reason they are also of much 

 value in structural botany, by the assistance they commonly 

 afford in enabling us to ascertain the relative succession and 

 position of the parts of the flower on the axis. 



The term anthcsis is sometimes used to indicate the period at 

 which the flower-bud opens. 



Besides the definite and constant relations which the parts of 

 the floral envelopes have to one another in the flower-bud, they 

 also have a determinate and constant relation in the same plant 

 to the axis upon which they are placed. In describing these 

 positions we use the terms anterior or inferior, siq^erior or 

 'posterior, and lateral. Thus, we call that organ 'posterior or 

 .nipcrior, which is turned towards the axis ; and that next the 

 bract from the axil of which it arises, inferior or anterior. 

 "V\lien there are four organs in a whorl, one will be superior, one 

 inferior, and two lateral, as in the calyx of Cruciferous Plants 

 {fig. 419). If there are five we have two arrangements. Thus, 

 in the calyx of the Pea or ler of plants {Legiiminosce). two sepals 

 are superior, two lateral, and one inferior ; while in the corolla one 

 petal is superior, two in f trior, and two lateral {figs. 435 and 461). 

 In plants of the Rose order {Rosacea), we have a precisely reverse 

 position exhibited by the parts of the two floral envelopes ; thus, 



