250 EVOLUTION OF PLANTS 
While at first sight it would seem that flowers having 
stamens and pistil together would usually be self-pol- 
linated, such is rarely the case, at least in showy flowers. 
An examination of these reveals many effective arrange- 
ments by which this is prevented and cross-fertilization 
made necessary. One of the commonest and simplest 
means is the maturing of the stamens and pistil at dif- 
ferent times. Usually it is the stamens which are ripe 
Fie. 50 (Cross-fértilization). — A, flower of Erodium, one of the Geranium 
family; the flower is inconspicuous and capable of self-pollination ; B, 
stamens and carpels of Erodium; the stigmas, s¢, are mature when the 
pollen is shed; C, young flower of Pelargonium; the pistil, », is im- 
mature; D, the pistil of an older flower which has shed the anthers; 
the stigmatic lobes, st, are now ready for pollination; E, young flower 
of a nasturtium (Tropcolum) ; three of the stamens are shedding their 
pollen and occupy the space in front of the opening of the spur; the 
other stamens are still closed, and with the immature pistil, st, are 
bent down; F, stamens and pistil of an old flower; the stamens have 
all shed their pollen, and the receptive stigma, st, now occupies the 
position in front of the opening of the spur; G, flower of broom 
(Sarothamnus) ; the stamens and pistil are included within the keel, 
k&; H, a flower which has had the keel forced down, liberating the 
stamens and pistil. 
first (proterandry), but proterogyry, or the earlier 
maturing of the pistil, may also occur, e.g. the common 
