40 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



petal law of expansion, but the whole mass of inflorescence 

 the centrifugal. 



317. This arises from the partial centripetal developement 

 commencing among the upper extremities of the inflorescence, 

 instead of among the lower. 



318. Consequently, this difference of expansion will indicate 

 whether a particular form of inflorescence proceeds from the 

 developement of the buds of a single branch, when it is called 

 simple, or not, when it is called compound. 



319. Whenever the order of expansion is centripetal, the 

 inflorescence is to be understood as simple ; when it is centri- 

 fugal, it is compound, although in appearance simple. This 

 difference is often of great importance. 



320. When the order of expansion is irregular, it indicates 

 that the mode of developement of the flowers is irregular also, 

 either on account of abortion or other causes. 



321. Sometimes all the flowers of the inflorescence are 

 abortive, and the ramifications, or the axis itself, assume a 

 twisted or spiral direction ; when this happens, a tendril is 

 formed; Ex. the Vine. 



X. FLORAL ENVELOPES. 



322. The Floral Envelopes are the parts which immediately 

 surround the sexual organs. 



323. They are formed of one or more whorls of bracts, and 

 are therefore modified leaves (274). 



324. In anatomical structure they do not essentially differ 

 from the leaves, farther than is necessarily consequent upon 

 the peculiar modifications of size or developement to which 

 they are subject. 



325. When the floral envelopes consist of but one whorl of 

 leaves, they are called calyx. 



326. When two or more whorls are developed, the outer is 

 called calyx, the inner corolla. 



