n 



VEGETATIVE REPRODUCTION. 253 



perianth becomes equal, and later is more rapid upon tin- 



inner face than the outer. At this time 



the flower unfolds, the perianth spreading 



more or less and exposing the stamens 



and pistils within. These variations in g ^^^ cd 



growth are often repeated, the stimulus 



being light or heat or both, when it is F i< ; 7 73 .-Poiienmassfroin 



necessary to protect the spores against ^ r s 



unfavorable weather. Such flowers open gj 



and close several times before their leaves 



wither. (See also f 286.) 



354. Calyx and corolla. The leavesof 



the perianth are usually arranged upon the torus in two or 

 more circles or in a low spiral. They may be all alike or 

 differentiated into two series, an outer and an inner. In the 

 latter case those of the outer row or rows constitute the calyx, 

 and the inner set the corolla. 



355. The calyx.- -The calyx leaves, or sepals, are generally 

 green and possess a great variety of form. When separate, 

 the sepals are usually sessile and broad, with more or less 

 pointed apex. The sepals are often apparently united in the 

 manner already described for the stamens, the originally 

 separate portions appearing as teeth or lobes at the rim of a 

 cup or tube, or some similar structure. Occasionally the 

 sepals are not persistent, but fall as the bud opens or shortly 

 thereafter. More commonly, however, the calyx, especially 

 when undivided, remains throughout the entire development 

 of the flower, and often of the fruit. 



356. The corolla.- -The inner set of perianth leaves, the 

 petals, constitutes the corolla. The corolla presents a greater 

 variety of form and color than does the calyx. The petals 

 may be sessile or have a short or long stalk (fig. 274). The 

 corolla may develop a cup or tube, as described for the calyx, 

 with teeth or lobes representing the petals (c, c, fig. 267). It 



