Growth and Movement. 107 



formation of starch and proteids, the production of secre- 

 tions, the growth of plasma membrane, and building of 

 cell wall. While it is not absolutely known that the parts 

 of the cell have the distinct functions here assigned to 

 them, yet the circumstantial evidence that they do amounts 

 almost to proof. 



We find, then, that we may look upon the Uving cells 

 of the plant body not only as units of structure, like the 

 bricks of a house, but as centers of vital activity which 

 induce and regulate whatever the plant does. 



80. Advantages of Cellular Structure. — The construction 

 of the plant body from many small cells has certain distinct 

 advantages : it renders the body stronger, lessens liability 

 to fatal injuries, and makes division of labor possible, so 

 that one part of the body may protect the other parts; 

 one part may give strength to the whole body, while other 

 parts may be adapted for transporting or manufacturing 

 materials, etc. Thus the business of the plant is economi- 

 cally and efficiently carried on. 



81. Continuity of Living Substance. — While the live 

 part of the plant body seems to be divided by the cell 

 walls into numberless units, it is probable that these are 

 united by minute strands of living substance, which are 

 difficult of demonstration by means of the microscope; 

 their existence, however, has actually been demonstrated 

 in many cases. We are justified in the conception that 

 the whole live body of the plant stands united by living 

 substance, from the farthest roots to the remotest buds, 

 although apparently severed by numberless cell walls. 



82. Cell Division. — The enlargement of the plant body 

 depends upon the multiplication and enlargement of its 

 cells. Multiplication of the cells is brought about by their 

 division, one cell becoming two by the formation of a new 



