FLOWER AND FRUIT 163 
seem to show that this is probably not absolutely true. 
The most that can with certainty be said is that in 
many plants cross-pollination produces a better crop 
of seeds and enhances the vigour of the offspring. It 
is obvious that there are two varieties of cross- 
pollination, first where the pollen of one flower reaches 
the stigma of another flower on the same »plant, and 
next where the pollen reaches the stigma of a flower 
on a different plant. The primrose seems to need the 
latter kind, though in most plants there is but little 
diminution in vigour if the former is effected. 
Self-pollination is effected in a great number of 
common plants. Wheat flourishes when subjected to 
it, and many common weeds, such as wire-weed or 
eommon knot-grass, chieckweed, and mallow con- 
stantly make use of it. The flowers of the geranium 
are generally self-pollinated, while, in the evening 
primrose, though cross-pollination is the rule, self- 
pollination often takes place, even before the flowers 
open. Cleistogamic or Cleistogamous (Gk. cleistos shut 
and gamos a marriage) flowers are those in which self- 
pollination takes place within the unopened bud. In 
the violet, a few weeks after it has apparently done 
flowering, there is produced a fresh crop of small 
colourless flowers borne on short stalks near the 
ground, or, in some cases, even underground. ‘These 
cleistogamous flowers never open but still produce 
fruit and seed. The petals, being no longer needed for 
the attraction of insects, almost disappear, and the 
anthers are so placed that they come into contact with 
the stigma as soon as it is ripe. In the thread-like 
violet of New Zealand (Viola filicaulis), about January 
small cleistogamic flowers are produced on short stalks 
less than an inch long. The style, which is as long as 
usual, is coiled on top of the ovary so that the stigma 
may come into contact with the anthers. It is. a rare 
thing for orchids to be self-pollinated, but in, 
