56 ELEMENTARY, BOTANY 
the same way flowers which as a rule are cross-pollinated 
may, in the absence of insect visits, be self-pollinated. 
Stellaria graminea, (a Chickweed), is an instance of this. This 
flower has ten stamens: first, the outer five open and the 
anthers become covered with pollen (Fig. 67); then the five 
inner stamens dehisce, and as those are withering the styles 
which before. had been bent inwards rise up, spreading out 
their stigmatic surfaces. The nectaries are green swellings on 
the base of the five outer stamens. If insects visit a flower in 
the condition drawn in Fig. 67, they must get pollen from the 
five outer stamens; if, however, the stamens have withered, 
then the insect will come in contact with the stigmas, and will 
deposit the pollen they have brought from some other flower. 
Supposing, however, that insect visits do not occur at all— 
and as the flower is inconspicuous that may often be the case 
—then the stigmas curling over come in contact with the 
anthers, and self-pollination ensues. 
In the present state of our knowledge, then, it may be 
asserted that the majority of flowers are usually cross-polli- 
nated; but that when cross-pollination cannot take place, 
owing to the absence of insect visits, self-pollination occurs 
more frequently than Darwin had reason to think. Further,. 
it may be stated that the power of self-pollination has been 
altogether lost by some plants which have become highly 
specialized-—as regards the colour: and form of flower—tfor 
cross-pollination. 
The putting When the alee grain reaches the stigma it 
outofthe puts out a tube which makes its way through 
Pollen Tube. the style into the ovary, and eventually enters 
the ovule by a passage between the coats of the ovule. (See 
Fig. 70.) Inthe Crocus this passage of the pollen tube takes one 
to three days; in Orchids several days, weeks, or even longer ; 
in Aloes and plants which flower but rarely, a year or more. 
Pollination includes not only the transference of the pollen 
grain from the stamens to the stigma, but also the growth of 
the pollen tube until it reaches the ovule. 
