'22 



BOTANY. 



24. After the formation of the cell-wall it generally 

 grows, and increases its surface and thickness. Usually the 

 surface-growth at first preponderates, afterward that in 

 thickness. Neither the one nor the other is uniform over 

 all points of the cell-wall, hence each cell during its growth 

 may also change its form. As the growth of the cell-wall is 

 directly dependent upon the protoplasm, it is clear that it 

 can continue only as long as the protoplasm is in contact 

 with its inner surface. In the 

 growth of the cell-wall the new 

 cellulose secreted by the protoplasm 

 is deposited between the molecules 

 of the membrane already formed. 

 When the new molecules are de- 

 posited between the previously 

 formed ones only in the plane of 

 the cell-wall, surface-growth takes 

 place ; but when the planes of de- 

 position of the new molecules lie at 

 right angles to the plane of the 

 cell-wall, increase in thickness is 

 the result ; when the molecules are 

 deposited in both planes, the wall 

 Fig. m-Diagrams to illustrate increases both iii surface and thick< 



tiie intercalary growth of OSdogo- 

 nium. A, internal ring of cellu- 

 lose secreted at /,- B, showing 

 the wav in which, by the horizon- . . 



tai splitting of the ring, the ceil is terminal or intercalary. In tne lor- 



25. Surface-growth may be 



mer case the growth is greatest at 

 some point on the surface, decreas- 

 ii in g in intensity on all sides. The 

 Modified from sa h. growing point thus comes to pro- 



ject as a point or knob, or it becomes the end of a cylindri- 

 cal sac. If several points of growth occur in a cell it may 

 become star-shaped, and by a continuation of the process 

 repeatedly branched. The typical form of intercalary 

 growth takes place in definite belts which surround the cell, 

 as is seen in (Edogonium (Fig. 10). The growth of the 

 whole of the side wall of a cylindrical cell, as in Spirogyra, 

 is also a form of intercalary growth. 



