35 



FOURTH GROUP. SEED-PLANTS. 



whorls containing the same number of members, usually from two to five, rarely 

 more. The structure and appearance of the two whorls is different in most Dicoty- 

 ledons and in many Monocotyledons ; the outer whorl, the calyx:, is composed of 



ff, 



IV 



FIG. 270. Chenopodium Qiu'noa. I IV development of the flower (longitudinal section) ; II the calyx with glandular 

 hairs It, a anthers, k k carpels, sk ovule, x apex, of the flowering axis. K transverse section of an anther with four pollen- 

 sacs on the connective on, highly magnified. 



usually smaller and coarser green leaves, the inner, the corolla, of usually larger leaves 

 of delicate structure and colourless or coloured; it is convenient however, as Payer 

 has suggested, to call the inner whorl the corolla, and the outer the calyx, even where 



both whorls have the same structure, for 

 the sake of greater brevity of expression, 

 and also because these structural differences 

 are not infrequently absent, the leaves of 

 both whorls being sepaloid, as in the Jun- 

 caceae, or both petaloid, as in the Liliaceae ; 

 in Helleborus, Aconitum, and others, the leaves 

 of the outer whorl only, the calyx, are peta- 

 loid, those of the inner, the corolla, have 

 been changed into nectaries. In many Dico- 

 tyledons the perianth is not formed of alter- 

 nate whorls, but of a few or more or even 

 many turns of a spiral arrangement of leaves, 

 the number of which is then usually large 

 or indefinite ; the outer (lower) leaves may 

 then be sepaloid, the inner only petaloid, 

 as in Opuntia, or they are all petaloid as 

 in Epiphyllum and Trollius, or there is -a 

 gradual passage from the sepaloid through 

 the petaloid to the staminal structure, as in 

 Nymphaea. 

 Sometimes in place of sepaline or petaline leaves for the envelopes of the flower 



FIG. 271. Longitudinal section of an inflorescence 

 of Taraxacum ojficinale partly diagrammatically repre- 

 sented. The flowering axis a is expanded at the summit b 

 and bears the ligulate flowers d\ the axis has a number of 

 involucral leaves c beneath the inflorescence. 



