76 



PLANT BIOLOGY 



2. Find several grains that have formed tubes. What is the 



color and shape of the tubes ? 



3. Make a drawing at least a half inch in diameter of a pollen 



grain before it has sprouted and a drawing of another grain 

 that has sprouted. Label pollen grain, pollen tube. 



89. Pollination, germination of pollen grains, and fertili- 

 zation. We have now learned that pollen by the process of 

 pollination is carried to the stigma of the pistil and adheres 

 to the stigma by a sticky substance which is easily seen on 

 the stigma of the Easter lily and often by hairs, also, as is the 

 case in the tulip and gladiolus. It has been proved that this 

 sticky substance contains sugar which together with other 

 materials furnishes food for the growth of pollen tubes (see 

 88). As each tube forms, it makes its way down through 

 the stigma and style (if present), and finally reaches an ovule 

 in the ovary. The tip of the tube now penetrates an opening 

 called the micropyle (Greek, mi cro = small -\-pula = gate way) 



in the ovule. Part of 

 the living substance of 

 the pollen grain now 

 unites with a part of 

 the living substance of 

 the ovule. This union 

 is known as fertilization. 

 After fertilization has 

 taken place the ovule 

 develops into a seed. 



90. The cellular na- 

 ture of pollen and ovules. 



(If flowers are studied 

 FIG. 27. Pollen grain of lily and the de- f ^ autumn it is siifr- 

 velopment of the pollen tube, highly mag- m tne autumn ; 

 nified. (After Strasburger.) gested that this Section 



\ sperm 

 /nuclei 



