CELL-DIVISION.- 



33 



universal method of new cell formation is by division ; the 

 original nucleus of the cell disappears and in its place 

 two closely-apposed nuclei appear, the protoplasm divides 

 into two equal or unequal portions, the plane of division 

 passing between the two nuclei, and along this plane a new 

 wall, is formed. In the higher plants the division-wall 

 between the two new cells is formed simultaneously at every 

 point, but in some of the fresh-water algse it commences as 

 a ring from the previously existing cell-walls, and grows 

 inwards. The above changes result in the formation of 

 two cells out of the one previously existing, each furnished 



Fig. 5. — {x 540). G/tf(K£7^j(Z ^(7/j/if£rOT(2/z'^(7, a minute fresh-water alga, show- 

 ing stages in the formation of a colony or cell-family In A the proto- 

 plasm (shaded portion) of an individual is becoming elliptical with a 

 slight constriction in the centre, preparatory to dividing into two indi- 

 viduals ; the thick mucilaginous cell-wall is lamellated. In B the 

 division is complete, and the two daughter-cells or individuals have 

 become elongated preparatory to further division. In C the four indi- 

 viduals forming the colony are spherical and in the vegetative condition. 

 (From Strasburger. ) 



like the original with protoplasm, nucleus, and cell-wall. At 

 first the two cells are only half the size of the parent cell, 

 but eventually grow to the same size. In this method 

 there is no contracting or rounding off, and only one of the 

 six sides of the wall of the cubical or oblong cell is new, 

 the remaining five having belonged to the parent cell. 

 The reproductive portion of a plant includes all those 



D 



