CYTOLOGY 



27 



deposition of pectin, which seems to be the principal substance in the cell 

 plate which is thus formed (Fig. 12). 



The cell plate first appears in the middle of the cell and extends sideways 

 until it meets the old walls, its extending edge being surrounded by a zone 

 of dense cytoplasm called the phragmoplast. Thus a pectic layer grows 

 across the cell, dividing it into two. This layer becomes the middle lamella 

 of the new cell wall. Its margin is at first separated from the middle lamella 

 of the parent cell wall by the thickness of the cellulose laver on that wall. 

 The margin of the new membrane thickens and vacuolates, the cellulose 

 wall with which it is in contact breaks down and the vacuolated margin then 



NUCLEAR 



MEMSRAN'P- 



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.;, ■■X\::[-\./ . :, 



<y>- 



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V-1 ^"^"■•'■'■^^^^tit^'-t-*' 



i'^'CELL PLATE - i/'^Vf ':? 

 CLEAVAG?.' ^-■: ■'--;■ 



NUCLEOLUS ' 



Fig. 12. — Viciafaba. Diagram to illustrate two successive phases, A and B, 

 in the development of the cell plate. {After Fraser and Snell.) 



joins the middle lamella of the old wall, the vacuole becoming part of the 

 intercellular space system and acquiring a lining of pectic substances. Lastlv, 

 cellulose layers are deposited on the new lamella, making up a new primary- 

 wall between the two portions of the divided cell. 



The spindle fibres disappear when the daughter nuclei obtain their 

 nuclear membranes, but it is possible that a part of them may persist through 

 the new cell wall and form the basis of the protoplasmic connections 



(Fig- 13)- 



In certain special cases, and especially among lower plants, cell division 

 begins at the periphery of the cell, not at the middle, and a cleavage furrow 

 runs inwards towards the centre, closing in like the iris diaphragm of a lens, 

 until the cell is cut in two. The wall material in this furrow is deposited in a 

 manner similar to the above. 



Cell division in the higher plants normally obeys certain principles which 

 have been called the Laws of Cell Division, though exceptions can be found 

 to each of them in special cases. They are not based upon anything specific 

 to the living cell, but are the expression of physical factors which govern 



