NATURE OF PLANTS 69 



being handled by the thin walled parenchyma, while non-dif- 

 fusible, albuminous substances can readily be transported from 

 cell to cell through the perforations of the sieve tubes. 



35. The Strengthening Tissues of the Stem. Attention may 

 be directed at this point to the perfection of the arrangements 

 that give rigidity to the stem. The delicate cells of very young 

 stems or of any other part of the plant are distended by fluids which 

 they absorb. In this way a very considerable force, known as 

 the turgor of the cell, is exerted that may amount to over 200 

 pounds to the square inch. This pressure at first gives suffi- 

 cient rigidity to the rapidly "elongating stem, but as it increases 

 in size and the strain upon it becomes more considerable, thick 

 walled collenchyma cells are formed that keep pace in their 

 growth with the elongation of the stem. When elongation 

 finally ceases, tough fibers of stereome appear which are able 

 to meet the increasing strain upon the stem due to the formation 

 and development of its various organs. These strengthening 

 tissues, with which must also be included the tissues of the xylem, 

 are arranged with the same mechanical effects as are employed 

 in the construction of buildings, bridges, etc. In bending a 

 beam the strain falls especially on the convex and concave sur- 

 faces. The cells will be compressed on the concave side and 

 stretched on the convex side, while the tissues in the center will 

 be subject to the least strain. For this reason girders are 

 strengthened on the surfaces receiving the strain and weakened 

 in the center (Fig. 44). So also given a certain amount of 



FIG. 44. Common form of girder. 



material this is more effectively distributed in the form of a 

 hollow column than in a solid column since less strain falls upon 

 the center of the column. Therefore in stems we find . the 

 strengthening tissue very economically distributed at the periph- 



