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CHAPTER II. 



FOEMATION OF CELLS — TISSUES. 



We have seen that every plant commences its life as a single 

 cell, sometimes a naked piece of protoplasm, as a gamete, or one 

 clothed with a ceU-wall, as most kinds of spore. It follows from 

 this that aU the cells of which the most complex plant body is 

 composed are derived from one of these by some process of multi- 

 plication. In this, as in all other vital processes, the protoplasm 

 is the active agent. In the production of the gamete or spore 

 itself we have to do with the formation of a new cell, not neces- 

 sarily by a process of multiplication ; while in the case of all 

 zygospores and oospores the new individual is the result of fusion 

 of cells, and therefore involves diminution instead of increase 

 of number. 



We may distinguish several different methods of cell forma- 

 tion, which can, however, be classed under the two heads men- 

 tioned above. First we will take those cases in which the new 

 cell results from the division of an antecedent cell, and after- 

 wards those in which no such division takes place. 



1. Formation of New Cells by Division of Antecedent 

 Cells. — This is by far the most frequent method. Usually a 

 cell divides into two, and a wall is generally immediately formed 

 between them. Less often a variable number of cells is formed 

 in the interior of a cell, known generally as the mother-cell, and 

 they become clothed with ceR- walls simrdtaneously or not at all. 

 The former is most common in vegetative, the latter in repro- 

 ductive processes. The two processes are termed cell-division 

 and free cell-formation. 



Cell- division. — We have pointed out that the division of the 

 cell is brought about by the protoplasm. The actual division 

 is preceded by a division of the nucleus, which may be direct 

 or indirect, the latter being much the more general. The 

 process of indirect nuclear division is known as IcaryoTeinesis. 



In the cases that have been most completely investigated 



VOL. I. X 



