394 A TEXT-BOOK OF BOTANY. 



is one mediane prophyllon, the first leaf of the perianth is placed 

 on the front, while the two succeeding leaves of the perianth 

 occupy a position of 120° from this (Fig. 254 j. When, on the 

 other hand, as in the dicotyledons with pentamerous flowers, 

 two fore-leaves are developed, the first floral leaf (sepal) is 

 situated obliquely above the last fore-leaf, usually on the frontal 

 part of the flower; the second sepal is directly behind the first 

 or diagonally opposite to it, the remaining three leaves (sepals) 

 occurring in a spiral of two-fifths (Fig. 280). Several deviations 

 from this type occur, as in Lobelia (Fig. 224), Polygala, etc. 



Two types of inflorescence are distinguished: (i) The in- 

 definite, in which the flowers open or develop in acropetalous 

 or centripetal succession, and (2) the definite, in wdiich the 

 flowers open in basipetalous or centrifugal succession. The in- 

 definite type of inflorescence is seldom terminated by an ex- 

 panded flower, and two classes of this type are distinguished : 

 (a) Those in which the flowers are pedicelled, as in the raceme 

 (Fig. 267) and umbel (Fig. 344), and (b) in which the flowers 

 are sessile, as in the spike (Fig. 230) and head (Fig. 228). 



The RACEME is a long inflorescence with pedicelled flowers, 

 which are frequently subtended by bracts (Figs. 224, 225, and 

 293). The CORYMB is a modified raceme in which the pedicels 

 of the basal flowers are much longer than those of the apical, 

 and thus the inflorescence looks like an umbel. In the milkweed 

 the flowers have pedicels of the same length which arise from the 

 apex of the shoot or peduncle, and this form of inflorescence is 

 known as an umbel. In the Umbelli ferae a flower cluster or 

 umbellet takes the place of the individual flowers of the umbel, 

 and is known as a compound umbel (Figs. 346-348). 



The spike is also generally a long inflorescence, the flowers 

 being sessile (Fig. 230, illus. 3), the secondary spikes in grasses 

 being known as spi relets. The spadix is a form of spike, 

 w^hich is readily distinguished by the fleshy stem, in which the 

 flowers are frequently deeply imbedded, and wdiich is frequently 

 surrounded by a large bract, the so-called spathe, as in Arissema. 

 The catkin is a kind of spike with small, often imperfect flowers, 

 which falls off as a whole, as in the staminate catkins of the 

 oak. The catkins are mostly decompound, and in some species 



