FLOWERS 



187 



FIG. 80. Pollen grains. 



(6) Place fresh pollen in a Petri dish and add a few drops 

 of dilute sugar and water or honey. Cover it to prevent 

 evaporation and examine it from time to time with the micro- 

 scope. Look for very fine threadlike growths which come 

 from the pollen grains. 

 If a drop of eosin or 

 methyl green is added, 

 they will become more 

 readily seen. 



These are pollen 

 tubes. When pollen 

 germinates on the 

 stigma of a flower of 

 the same kind, the 

 pollen tube penetrates 

 the stigma, forces its 

 way down through the spongy tissue of the style, until it 

 reaches the ovary, and at last penetrates the micropyle of 

 the ovule. 



When this occurs, the living contents of the pollen grain 

 fuse with that of the ovule, and the latter is then said to 

 be fertilized. Hence this process is known as fertilization of 

 the ovule. 



It is not until this has occurred that the ovule begins to 

 grow. With few exceptions, only fertilized ovules ever 

 develop into seeds. 



Note. Since different pollens require different strengths of 

 sugar, it is well in making this experiment to sow the pollen in 

 Petri dishes containing sugar solutions of from 1 per cent to 15 per 

 cent strength. 



Definition. In order that the ovules may grow they must 

 receive pollen from the stamens. This is called pollination. 



