226 



A TEXTBOOK OF BOTANY 



[Ch. V, 4 



new tips and hairs have again made connection with the 



water supply. 



Each root hair is a cell, 

 possessing a cellulose wall 

 lined by living protoplasm 

 (Fig. 165) and a sap con- 

 taining various substances, 

 especially sugar, in solution. 

 The hairs are in close con- 

 tact with particles of soil, 

 and bathed in the surround- 

 ing water (Fig. 169). In 

 the root they are in con- 

 tact with the cortical cells, 

 which likewise have cellu- 

 lose walls, protoplasmic 

 linings, and sugar-contain- 

 ing sap; and the cortical 

 cells in turn are in contact 

 with the ducts which have 

 no protoplasmic linings. A 

 typical example of this 

 absorbing system is shown 

 by an earlier picture (Fig. 

 159), while its mechanical 

 construction is illustrated 

 by our diagrammatic Fig- 

 ure 166. 



The water in the ducts, 

 while sometimes containing 

 sugar and the like, is ordi- 

 narily nothing other than 



Fig 167.-- A pressure gauge at- u t ^^ some min . 

 tached to a root for the measurement 



of sap-pressure ; x J. eral matters in solution. 



The rise of the mercury in the long Furthermore this wate r is 

 tube above the level in the reservoir bulb § ' 



gives the sap-pressure in "atmospheres." forced into the ducts by 



