24 Plant Physiology 



indiscriminately applied to the liquid contents occurring 

 in the vacuolate areas large and small. 



The vacuoles are unquestionably of much physiological 

 significance, and certain materials diffuse into such spaces 

 and may to a considerable extent accumulate there. 

 The substances held in solution may include a variety of 

 organic or inorganic compounds, again referred to in the 

 discussion of metabolic products. In some cases the color 

 of plants is due to coloring matters occurring in the cell- 

 sap alone. With respect to the other liquid cell contents 

 the vacuoles may have therefore a certain differential 

 character. 



17. Cell-forms. - Young or growing cells in tissues are 

 often somewhat rectangular in outline. When, however, 

 the pressure of adjacent units is released, there is an ap- 

 parent tendency to assume a form more or less spherical. 

 This should not be confused with the fact that micro- 

 organisms possess a considerable diversity with respect to 

 their specific forms ; indeed many unicellular organisms 

 of elongate forms which grow for a time in pairs or groups 

 become also more convex along the lines of attachment 

 upon being set free ; thus rod-shaped bacteria may become 

 more rounded at the ends. In the meristem of the grow- 

 ing tip "where the cells are closely united, and encompassed 

 by a variety of pressures, the typical form of the cell is 

 isodiametric or polyhedral. Back of the formative region, 

 under the influence of the growth pressures and various 

 other stimuli, there is a tendency for many cells or cell- 

 groups to take up an elongate form. The latter may 

 make possible, in time, other important modifications. 



Under all circumstances the embryonic meristem cell 



