IIG 



rilK FLOWER 



PROCESSES LEADING TO THE FORMATION OF SEED 



224. The student i,s alreai.l}- aware that the pollen is 

 destined to reach the stiyniatic surt'aee of the pistil ; and 

 he prol.)ahly also understands in a general way that the 

 result of the pollination iif a fl(.)\\er 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 oi the pollen from its deposition on 

 the stigma ( pollinatiou) onward and the resulting effect 

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



225. The pollen grain has been briefly described as a 

 simple vesicle filled with living nuitter, capable of growth. 

 The wall is relatively strong, though thin and transparent, 

 and often beset with projections. Tlie living substance 

 within, termed protoplasm, is more or less jellylike in 



consistency and clearness, 

 but is far from Ijeing 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. 

 Tliese members may be 

 dimlj' made out in the living 

 protoplasm with the com- 

 pound microscope. But when killed and stained with 

 proper dyes, the structure stands out witli distinctness and 

 its great complication is then seen. A constant com- 

 ponent is a rounded central body of especially dense proto- 

 plasm, Ivuown as the nucleus (Fig. 1<]3). In the earlier 

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

 pollen grain is tlien an excellent example of the typical 

 vegetable cell. 



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

 of minute memljers, or cells, essentially similar to the 



1G3. A pollen i:rain highly magnified. 

 It oniit:xiii.s two nuclei (n, »') 

 at the stage here represented. 



