78 APPLIED BIOLOGY 



happen when a bee leaves a flower and at once goes back to 

 it, carrying pollen-dust received on its first visit. 



It seems probable from the arrangement of the bean 

 flower that insect visits are necessary to distribute the pollen- 

 dust of this kind of flower. Darwin, the famous English 

 biologist, made many experiments by keeping beans covered 

 with netting so that insects could not reach the flowers, and 

 the result was that seeds rarely formed. Pea flowers are 

 very similar to those of the bean, but the stigma is so near the 

 anthers that it often gets pollen-dust before the flower is 

 visited by insects. Botanists call such a flower self -pollinated. 

 The bean, then, is not often self-pollinated, but cross-pollina- 

 tion (meaning pollen from other flowers) usually occurs. 



We now see the significance of the peculiar irregular 

 arrangement of the petals of the bean flower : the three 

 big petals are arranged so as to make bees or similar insects 

 alight in a certain way; and the other petals form a pro- 

 tective covering for the stamens and pistil, and at the same 

 time are a curious mechanism for preventing pollen-dust 

 from reaching the stigma of the same flower, thus insuring 

 cross-pollination. 



Many plants closely related to the bean clover, locust, 

 pea, wistaria, peanut, are common examples have similar 

 flowers. If opportunity offers, observe insects visiting such 

 flowers. There are many other types of flowers arranged or 

 adapted for the transfer of pollen-dust by insects, and some 

 of these will be described in the lesson on flowers ( 192). 



Ovules. (D) Carefully remove the corolla and thus expose 

 the pistil of a bean flower. Hold up to the light and notice a row 

 of opaque spots in the ovary. Then with a sharp knife or a razor 

 split the ovary lengthwise. A low-power miscroscope or a hand- 

 lens will make clear that the opaque spots are "seeds." In this 

 early stage, however, the term ovule should be applied to each one 

 of these structures which later grows into a seed. Inside each 

 ovule an embryo develops, and later when the seed sprouts the 

 embryo grows into a new plant. 



