ABSORPTION OF LIQUIDS 183 



254. Tensions of the Tissues. The wood and other dead 

 tissues in a stem are not turgescent and hence are in a pas- 

 sive condition. The pith and cortex are usually not equally 

 turgescent, so that a number of tensions or strains are set up 

 in stems where these tissues are bound together. If the stem 

 is separated into its different tissues they will lengthen or 

 shorten accordingly as they have been compressed or stretched 

 in the plant. These tensions are also exerted in radial and 

 tangential directions. 



255. Longitudinal Tensions. Secure young and rapidly grow- 

 ing tips of stems of Sambticus, Nicotiana, or Helianthus, or flower 

 scapes of Taraxacum. Split into quarters and note the positions 

 assumed. Separate the pith, wood and cortex of a portion of a 

 stem of Helianthus 50 cm. long with a sharp knife. Measure the 

 length of the stem accurately before doing so, and then find exact 

 length of the pith, wood and cortex, a few minutes later. The pith 

 may extend to 54 or even 56 cm., while the wood and cortex 

 will change but slightly. 



256. Absorption of Liquids. Every living cell exerts its own 

 osmotic effect upon fluids with which it comes into contact. 

 These fluids may permeate the wall only, and it is possible for a 

 substance to traverse the entire length and thickness of the body 

 of a plant in this manner, or they may penetrate the plasmatic 

 and vascular membranes. The living cell differs in its action from 

 the osmometer in two particulars : First, the substances attracted 

 into the cell may be converted into solid or non-diffusible form 

 leaving the original attractive compounds to draw in a new supply 

 of the same substances, continually; secondly the plasmatic 

 membrane is controlled in such manner that it allows the diffu- 

 sion of some substances and bars others, in a manner not explain- 

 able by purely physical laws ; the relations of the plasmatic mem- 

 brane to any given substance may change from time to time also. 

 The so-called selective power of plants rests upon these facts. By 

 this faculty different cells and different species take from the sub- 

 stratum entirely different substances. 



