THE CELL-WALL 



like wood and bast fibers, is approximately .03 mm., or about 

 one-fourth the thickness of one of these pages. Several reasons 

 can be suggested for this habit- 

 ually small size: (a) The plant 

 is made stronger thereby. The 

 cell-walls 'must be, on the whole, 

 extremely thin to permit an easy 

 interchange of materials, and the 

 smaller the cells the stronger they 

 will be with a given thickness of 

 wall. The smaller the cells the 

 greater the number of walls in a 

 given volume of tissue, so that the 

 whole tissue is made stronger. 



(6) Each cell is a chemical labor- 

 atory, each nucleus a center of oxi- 

 dations, so that the more there 

 are of these the greater the activity 

 of the whole body. (c) The 

 smaller the cells the greater the 

 amount of protoplastic surface ex- 

 posed for the osmotic interchange 



of materials between contiguous 



cells and between cells and inter- 

 cellular spaces, (d) The smaller 



the cells the greater the number 



of nuclei to send forth hereditary 



stimuli which must dominate 



every part of the body. All of 



these problems are of great 



moment to the well-being of 



plants, and the minute size of the 



cells is one of the factors in their 



solution. 



The Cell-wall. The cell-wall is the skeleton of the proto- 

 plast, preserving its form and protecting it from danger; it also 



FIG. 8. i, cells from a cross section, 

 and 2, from a longitudinal section, 

 through the primordial meristem of 

 the growing apex of Aristolochia 

 sipho; the cells are essentially alike 

 from both points of view. 3, 4 and 5 

 show cells from some of the different 

 tissues which the primordial meristem 

 produces. Note the different shapes 

 and thicknesses of walls. 3 is from 

 the sclerenchyma ring; 4, from the 

 collenchyma, and 5, from the epider- 

 mis. All magnified to the same scale. 



