no Introduction to Botany. 



the walls become stretched, by the laying down of new 

 materials in and about the walls, and by the addition of 

 appropriate materials to the different parts of the proto- 

 plast. The addition to the cell wall is evidently accom- 

 plished by the plasma membrane, while each part of the 

 protoplast is presumably capable of assimilating materials 

 for itself. We see how necessary water is to growth, not 

 only in giving up its hydrogen and oxygen for the manu- 

 facture of plant substance, or in acting as a solvent, but in 

 supplying the necessary stretching force which initiates the 

 first step in increase in size. 



84. Changes in Character of Cells. — As the cells attain 

 their definite size and form, we find that they do not all 

 behave in the same manner, although they have all de- 

 scended from a common parent cell. Some become long 

 and fibrous, while others remain nearly isodiametric ; some 

 fuse end to end to form water tubes, while others infiltrate 

 their walls with waxy substances to keep water from pass- 

 ing through them. The cells have come to act so differently 

 because they have been under different conditions. Those 

 which are on the outside exposed to the air have not the 

 same surroundings as those which lie at the interior, and 

 the various zones of interior cells are under different con- 

 ditions of exposure to air, and of tension, pressure, etc. 

 Thus the unUke action of cells which have descended from 

 a common ancestor may be in part acounted for. 



85. Regions of Continued Growth. — In dicotyledonous 

 plants, such as the oak, maple, elm, etc. {B, Fig. 47), the 

 regions of continued growth lie at the apices of roots and 

 shoots, in the cambium zone which separates the bark from 

 the wood, and in certain zones of cells called cork cambium 

 which give rise to the cork of the bark. In monocoty- 

 ledonous plants, such as grasses {A), Aloe (C), etc., there 



