GROWTH OF PLANTS 115 



mature, its growth ceases. The annual plant may show a similar 

 tendency. But in the perennial plant, growth continues through- 

 out its life and consists in a continuous formation of new organs 

 from special meristematic regions. Young tissue is always 

 found even in trees several thousand years old; while, in an old 

 animal, the entire organism in all of its parts is senescent. In 

 the animal, only a few parts represent dead tissues; while in 

 plants, old dead structures may still function for support, pro- 

 tection, and water conduction. 



27. Growth of Separate Cells and the Processes Involved. 

 Three Stages of Growth: Embryonic Development, Expansion, 

 and Internal Differentiation. — Since the growth of separate 

 organs as well as of entire plants is made up of the sum of the 

 growth of the separate cells composing them, it is essential for 

 understanding the laws of growth to study the alterations that 

 occur during growth in the cells of the growing points and 

 meristematic tissues. Meristematic cells are comparatively 

 small, have very thin cell walls, and are filled with protoplasm 

 with a large nucleus. Growth of meristematic tissues proceeds 

 as follows. The amount of protoplasm in the cells gradually 

 increases, resulting in an increase in size of the cells themselves; 

 thereupon, division of the nucleus and of the cytoplasm takes 

 place, and the newly formed cells are separated from each other 

 by new walls. The accumulation of living substances begins 

 anew, followed by a new division of cells. Consequently, in 

 the meristematic regions, the fundamental process is an increase 

 in the number of cells and in the general mass of living 

 matter. 



In organs with uninterrupted growth, such as roots or young 

 stems, the total mass of meristematic tissues remains constant 

 in spite of the continuous division of its cells. This is due to 

 the fact that in the lower part of the growing region the meriste- 

 matic cells pass into the second stage of growth, elongation. This 

 stage is characterized by the appearance of vacuoles in the proto- 

 plasm, which become filled with the cell sap (Fig. 23, lower and 

 central cell). The vacuoles enlarge rapidly, and finally the 

 protoplasm is only a thin lining appressed to the walls. The 

 vacuole of the cell has increased, the walls have grown consider- 

 ably, but the amount of protoplasm apparently remains about 

 the same as in meristematic cells (Fig. 23). 



