144 PLANT PHYSIOLOGY 



ascent of the water; then wholly immerse them for a time and measure 

 any change of dimensions, (b) Prepare a smooth piece of soft wood, 

 some 5X5 X-5 cm., and measure, preferably with vernier calipers, its 

 exact dimensions, cover one face with wet filter-paper until it warps, 

 then note the force needed to flatten it, which will give some rough 

 measure of its power of absorption; immerse it wholly in water until 

 saturated and remeasure the dimensions; then dry it thoroughly 

 and remeasure. (c) Select a hygroscopic awn of Erodium, Avena, or 

 Stipa, and mount it vertically upon a cork by aid of sealing-wax ; bring 

 it into places of various humidities, from the dryest possible to satura- 

 tion, and observe the resultant movements, (d) Mount a thin section 

 of dry Laminaria stem (or cross-sections of coats of flaxseed) in 

 strongest alcohol for the microscope, and, with the object under obser- 

 vation, draw in water to gradually replace the alcohol, and observe 

 result. 



In connection with these experiments the student should 

 now make himself acquainted with the physical nature of dif- 

 fusion, especially with respect to the molecular state of the solute, 

 the energy producing the diffusion movement, the diffusive capac- 

 ity of the various common substances (salt, sugars, potassium 

 nitrate), the distinction between crystalloids and colloids, the 

 nature of imbibition by membranes with the molecular states and 

 energy involved and its relation to capillarity, Nageli's micellar 

 theory of membrane structure and water movement therein, and 

 the physical basis of hygroscopic movements of dry tissues. Then 

 he should proceed to the study of osmosis proper, which he may 

 well approach thus: 



What are the actual phenomena of osmosis? 



Experiment. Into an osmoscope, preferably one with a cup 

 membrane for rapid action, pour a suitable quantity of a strong solu- 

 tion of sugar, for which molasses is very convenient. Support the 

 instrument upright with the membrane in pure water, and observe 

 the result. 



Also test the result when water is placed inside the osmoscope 

 and the solution outside. 



Also observe the result when two different strengths of the same 

 solution are used. 



Osmoscopes and Osmometees. An osmoscope consists essentially 

 of an open tube over one end of which is tightly tied a membrane, the given 

 solution being placed in the tube with the membrane immersed in water. 

 Since, however, the visibility of the osmotic movement, as measured by 

 rise of liquid in the tube, is directly proportional to the size of the mem- 



