292 EXPERIMENTS WITH PLANTS 



ers so treated are able to set seed? We must, of 

 course, have unmutilated flowers (if possible on the 

 same plant) for comparison, and it is important that 

 we choose a plant whose flowers set seed' freely. 



The effectiveness of the stigma in retaining pollen 

 may be tested by dusting the flower with flour or any 

 white powder and afterward endeavoring to blow it 

 away. In the Monkey- flower the stigma has two lips 

 which come together and hold the pollen firmly when 

 it has been deposited. Touching the open lips with 

 the point of a pencil will cause them to close at once. 

 The silk of the Corn is practically a long stigma 

 covered with projections to retain the pollen, which is 

 carried by the wind: Wheat and Grasses generally 

 have such stigmas, but they are much shorter and less 

 conspicuous than in the Corn. 



It is the work of the stigma not only to capture and 

 retain the pollen, but also to provide favorable con- 

 ditions for its germination and to nourish it during the 

 growth of the pollen -tube. The sticky substance on 

 the stigma contains sugar (as you can easily ascertain 

 by tasting or applying a chemical test) , which serves 

 the germinating pollen -grain for food. If we place 

 pollen- grains in a drop of sugar solution of the right 

 strength, we shall be able to observe their germination, 

 which may, in some cases, take place in a few minutes 

 (Willow, Sweet Pea) . It is advisable to make up cane 

 sugar solutions of 35 per cent, 10 per cent and 5 per 



