168 ELEMENTS OP STRUCTURAL BOTANY. 



enlarges and divides again, and so the process goes on.. 

 When cell-division takes place in one direction only 

 filaments or threads are formed ; if in two directions,, 

 surfaces are formed ; while division in three directions 

 gives rise to masses. 



It is evident that every part of a plant, however much 

 altered in its later history, must in its earlier stages have 

 consisted of this thin-walled cellular substance, or 

 meristem, as it is called from its power of dividing. 



283. Cell-division, then, is the method of new cell 

 formation which prevails in the vegetative parts of the 

 higher plants. In the production of pollen, however, and 

 of the spores of vascular cryptogams, four new nuclei are 

 formed in the cell, and the protoplasm collects about 

 these, eventually secreting walls, so that four new and 

 complete cells are formed within the original one, and 

 these sooner or later make their escape. This mode is known 

 as free cell-formation. In the production of the endo- 

 sperm cells in the embryo-sac and the spores of many of the 

 lower plants a similar process goes on ; but here the 

 division of the nucleus is not limited to four portions, as 

 in the cases just mentioned, but may be carried on to an 

 indefinite extent. 



284. In some lower plants the entire contents of two 

 adjacent cells may coalesce to form a single new cell. 

 This mode is known as conjugation. Also, the contents 

 of a cell may contract and develope a new cell- wall, a 

 process known as the rejuvenescence or renewal of a cell. 



285. Tissues. An aggregation of similar cells is 

 Called a tissue. Originally, every part of a plant consists 

 oi meristem, that is, of cells capable of dividing. But 



