POLLINATION AND FERTILIZATION 118 
Arumand Aristolochia. Geranium, Carnation and Sun- 
flower are good examples of protandrous flowers. 
(c) Heterostylism—A curious phenomenon found in 
some flowers is that of HuTERostyLisM. It is well seen 
in Oxalis. This plant bears three kinds of flowers 
(TRIMORPHIC), each of which has different lengths of sta- 
mens and styles (figs. 62, 68). Complete fertility is only 
obtained when the pollen from a long stamen is taken 
to a long-styled flower, or from short to short. The 
Fis. 62.--Sections of Prim- Fie. 63. — Trimorphic 
rose Flowers, Showizg Di- Flowers of Oxalis, Show- 
morphic Heterostylism. ing Stamens and Pistil. 
a, Long-styled Primrose. 06. 
Short-styled Primrose. 
short-styled forms can, of course, be self-pollinated, but 
this results in few seeds, which are more or less sterile. 
Further details are given in the description of this 
flower (see § 152). The Primrose bears DimorpHic 
flowers, ie. one flower has a long style and stamens 
half-way down the corolla tube, the other has a short 
style and stamens at the top of the tube (see fig. 62). 
(d) Seif-sterility.—Another condition preventing self- 
pollination is SELF-STERILITY, i.e. incapability of a flower 
to be fertilized by its own pollen. Thisis found in some 
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