116 



THE FLOWER 



PROCESSES LEADING TO THE FORMATION OF SEED 



224. The student is already aware that the pollen is 

 destined to reach the stigmatic surface of the pistil ; and 

 he probably also understands in a general way that the 

 result of the pollination of a flower is the production of 

 seed ; that if pollination fails to be brought about, the 

 ovules of the unpollinated pistil do not develop into fertile 

 seed. The history of the pollen from its deposition on 

 the stigma (^pollination') onward and the resulting effect 

 on the ovule (^fertilization) are now to be followed. 



225. The pollen grain lias been briefly described as a 

 simple vesicle filled with living matter, capable of growth. 

 The wall is relatively strong, though thin and transparent, 

 and often beset with projections. The living substance 

 within, termed frotoplamn^ is more or less jellylike in 



consistency and clearness, 

 but is far from being a 

 simple mass of jelly. The 

 protoplasmic body is in fact 

 very definitely and highly 

 organized, with permanent 

 parts or organs performing 

 definite functions in har- 

 mony with one another. 

 These members may be 

 diml}^ made out in the living 

 protoplasm with the com- 

 pound microscope. But when killed and stained with 

 proper dyes, the structure stands out with distinctness and 

 its great complication is then seen. A constant com- 

 ponent is a rounded central body of especially dense proto- 

 plasm, known as the nucleus (Fig. 163). In the earlier 

 stages of the pollen grain there is but one nucleus. The 

 pollen grain is then an excellent example of the typical 

 vegetable cell. 



226. Cellular structure of plants. — Every plant is made 

 of minute members, or cells, essentially similar to the 



163. 



A pollen grain highly magnified. 

 It contains two nuclei (n, n') 

 at the stage here represented. 



