50 ELEMENTARY BOTANY 
CHAPTER bx. 
POLLINATION 
Gross-ang LN order that flowers may make seed, the pollen = 
Self- from the stamen must reach the stigma. The 
aes pollen of a flower may fall on the stigma of the 
same flower, or pollen from one flower may be carried by 
some agent to another flower of the same species. In the 
former case the flower is self-pollinated ; in the latter, cross- 
pollinated. 
At first sight one is inclined to wonder why pollen should 
be carried from one flower to another, as the waste must be 
great, and it would seem more natural for flowers to pollinate 
themselves ; but Darwin has shown that the waste of pollen is 
more than counterbalanced by an undoubted advantage. For 
some eleven years he made exact observations, in order to 
try and ascertain the relative value of cross- and self-pollina- . 
tion. He raised more than a thousand plants from seed which 
had been produced in some instances by cross-pollination, in 
others by self-pollination, in order to compare the results of 
_ the two processes in the same species, and he came to the 
conclusion that plants produced by cross-pollination had the 
advantage over those produced by self-pollination in being 
more hardy and better able to-stand unfavourable conditions. 
Even where a species is usually self-pollinated, as in the bee 
orchid, it is, Darwin thought, occasionally cross-pollinated. 
Instances of _(”) _Cross-pollination must inevitably take place 
Cross- in those plants in which the flowers have stamens 
re carpels, but not both. In the willow (Plate IIT, 
Figs. 55, 56) the staminate flowers are:borne on one tree, the 
pistillate on another. These latter have nectaries situated 
~ between the ovary and the bract of a flower. They are 
readily visited by insects, for, although they have no corollas, 
