VEGE TA TI VE REP ROD UCl'ION. 



205 



292. Perianth. — The perianth is not present in any 

 gymnosperms (^ 281), except in a rudimentary form in a 

 few species of the highest order. In angiosperms the 



B C b E b 



Fig, 166.— Pollen grains. -4 , white water lily (iVj/?K>A<iffa alba). B, a thistle {Cirsium 

 nemorale), C, a mallow {Hibiscus ternatus). Z>, dandelion {Taraxacum offi- 

 cinale). Magnified 200 diam. — After Kerner. E^ pine, showing bladdery enlarge- 

 ments, 3, ^, of the outer layer of the cell-wall. Magnified 400 diam. — After Sttas- 

 burger. 



perianth, which is rarely wanting, is primarily for the protec- 

 tion of the stamens and pistils. As in all cases where leaves 

 are produced rapidly and close together on a short axis, they 

 grow during their early stages more 

 rapidly upon the outer face than the 

 inner. They are, therefore, concave in- 

 ward and closely pressed together, form- 

 ing a bud. At a certain stage the growth 

 upon the two faces becomes equal, and 

 later is more rapid upon the 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 growth are 

 often repeated, the stimulus being light 

 or heat or both, when it is necessary to protect the spores 

 against unfavorable weather. Such flowers open and close 

 several times before their leaves wither. (See also \ 244.) 



I'lG. 167. — Pollen mass 

 from an orchid. The 

 pollengiains are arranged 

 in packets, j>, which are 

 aggregated at the end of 

 a stalk, cd^ terminating in 

 an enlarged sticky disk, 

 ^, by means of which the 

 pollen mass adheres to 

 insects. Magnified about 

 10 diam. — A^er Engler. 



