OR, MANUAI, Olf YHB APIARY. 



187 



seeds. In Chinese wistaria, Wistaria sinensis, there are, says 

 Goodale, about 7,000 pollen-grains to each ovule. The color of 

 pollen is usually yellow ; but we often find it orange, reddish, 

 nearly white, and in several Gilias in California it is bright 

 blue. Pollen-grains are really single cells, and have two 

 coats ; the outer is the extine, which may be smooth, variously 

 sculptured, or even thickly set with spines (Fig. 81). These 

 spines, as also the color, often enable us to tell the species of 

 plant from which the pollen came. Usually the extine is per- 



FiG. 81. 



Pollen-Orains, from A . I. Jioot Go. 



forated, though the inner wall — intine — is not. These perfora- 

 tions are also definite in number within the species. These 

 holes give opportunity for the pollen-tubes (Fig. 2S2, T) to push 

 out after the pollen-grain reaches the stigma of the flower. 

 Where there are no perforations of the extine, the wall breaks. 

 In some cases like orchids, pollen-grains are held together by 

 an adhesive substance. In our milkweeds we notice a similar 

 grouping of pollen-grains (Fig. 227) which often are very dis- 

 turbing to bees and other insects. 



The composition of pollen, says Goodale, is protoplasmic 



