1 84 MORPHOLOGY AND LIFE HISTORY 



presides over the development of the pollen- 

 tube. The smaller is the generative nucleus. 



(b) Make drawings, 2 cm. in diameter, of a micro- 

 spore and of a male gametophyte, labeling all 

 parts. 



(c) After pollination, the formation of the pollen- 

 tube takes place. This is usually spoken of as 

 the "germination of the pollen-grain." 



The tube emerges through one of several weak 

 places in the wall of the grain, grows down 

 through the tissues of the style, digesting its 

 channel as it proceeds, or, in some species, fol- 

 lowing a canal already formed through the 

 style. 



The generative cell (generative nucleus with its 

 own protoplasm) follows down the pollen-tube, 

 and divides into two non-motile sperm cells. 

 In some species, e.g., Sambucus, (elder), this 

 division occurs before the tube develops. The 

 pollen-tube passes through the micropyle, and 

 discharges the sperm-cells near the egg. One 

 of the sperm-nuclei fuses with the egg-nucleus, 

 thus effecting fertilization. 



(d) For convenience in handling material the 

 observation of the finer structure of the anther 

 and pollen will be deferred until after the 

 microscopic study of the ovary and ovules (3, 

 below). 



3. Megasporophylls and Megasporangia. With a sharp 

 scalpel or razor make a median cross-section of the 

 ovary and observe: 



(a) Its outline. Each of the lobes represents the 

 section of one of the carpels (megasporophylls) , 



