THE FLOWER 



117 



pollen grain in internal constitntion, though of course 

 not as to form and external appearance. The cells of 

 vegetable tissue 

 take on various 

 shapes. Generally 

 their duration as 

 living elements is 

 limited. The walls . 

 become thickened 

 and hardened and 

 remain, after the 

 death of the cells, 

 as components of 

 the plant's frame- 

 work (e.ff. the fibers 

 of wood) . The 

 simplest plants 

 among the crj'iito- 

 gams consist of but 

 a single cell. 



227. The pollen 

 grain a plant. — 

 In ti'uth the pollen 

 grain itself behaves 

 like a simple plant. 

 For it absorbs water 

 and . nutriment 

 from the pistil 

 upon which it is 

 deposited, and uses 

 these materials in 

 growth. 



228. Growth is 

 manifested in two 

 ways: (1) in the 

 formation of new 

 nuclei in the proto- 

 plasm ; and (2) in 



164. 



Fcirtilizatiuu o[ the ovule. The pdUeii tubes 

 traverse the loose tissue of the stigma luid 

 style, finally emerging in the cavity ol' the 

 ovary. In tlie figure a tube is represented 

 as applying itself to the mieropyle of an 

 ovule. This ovule is seen in section, and 

 shows at the micropylar end the embryo- 

 sac with several nuclei, one of which takes 

 part in the formation of the embryo. 



the extension of the wall in a tube 



