78 SUPPORT OF THE STEM 



stances received from the root hairs to all parts of the plant 

 body quickly and with a minimum expenditure of energy because 

 of the elongated character of its cells and the absorption of many 

 of the cross walls of its cells. So the manufactured foods lo- 

 cated at any point in stem or leaf are gathered up and dis- 

 tributed by the phloem, the more soluble and diffusible substances 

 being handled in part by the thin-walled parenchyma, while non- 

 diffusible, albuminous substances can readily be transported from 

 cell to cell through the perforations of the sieve tubes. 



36. 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 



Fig. 45. Common form of girder. 



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 surfaces. 

 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 disturbance. Foir this reason girders are strength- 



