388 



SCIENCE 



[N. S. Vol. XLV. No. 1164 



the earth's surface, kept going through the 

 running down of energy and its escape from 

 the earth," and they remark further that this 

 view of the significance of the metamorphic 

 cycle involves a slight modification of the 

 prevalent interpretation of Hutton's law of 

 uniformitarianism in that while the same 

 series of processes are operating to-day as in 

 the past they are now working on different 

 propositions and distributions of substances 

 than formerly, with consequently slightly dif- 

 ferent results. 



A fourth part of the book is devoted to sug- 

 gestions as to laboratory work in metamorph- 

 ism, which by reason of its very general char- 

 acter appears to have been prepared as sug- 

 gestions for instructors of laboratory students 

 rather than for the students themselves. The 

 book is a valuable contribution to the broad 

 geological problems connected with changes of 

 all kinds which take place in rocks, but its 

 title is somewhat misleading. 



J. P. Iddings 



SPECIAL ARTICLES 



A PLANT MEMBRANE FOR DEMONSTRATING 

 OSMOSIS 



The writer has noted with interest that the 

 authors of recently published text-books in 

 botany are still advocating the use of egg 

 membranes and parchment membranes for the 

 demonstration of osmosis. It is unfortunate 

 that botany teachers should limit themselves 

 to animal membranes, parchment membranes, 

 or celloidin membranes in demonstrating to 

 students this very important phenomenon in 

 plant physiology. This is especially true when 

 we have readily available a natural plant 

 membrane which serves the purpose admirably. 

 I refer to the testa of the lima bean. 



Osterhout^ has suggested the use of the 

 testa of the lima bean in some osmosis experi- 

 ments of rather limited visual value. In the 

 botanical laboratories of Kansas University 

 Professor W. 0. Stevens^ has used this mem- 



1 Osterhout, W. J. V., ' ' Experiments with 

 Plants," 1906. 



2 Shull, C. A., ' ' Semipermeability of Seed 

 Coats," Bot. Gaz., LVI., 183, 1913. 



brane in the type of osmosis demonstration 

 providing for a rise of the more rapidly dif- 

 fusing liquid in a glass tube of narrow diam- 

 eter. In our own classes we have found the 

 experiment so satisfactory that the method is 

 here presented in detail. 



Two days before the experiment is to be set 

 up, place a number of clean lima beans on 

 clean moist paper or absorbent cotton in a 

 glass jar and cover with a glass plate. As 

 germination progresses some of the seed coats 

 will split almost as soon as swelling begins. 

 Others will stretch greatly without splitting. 

 The latter will best serve for the experiment. 

 It is also important to discard any which show 

 signs of bacterial or mold activity. After se- 

 lecting the bean to be used carefully split the 

 testes through the micropyle and hilium and 

 remove the two halves. Each will serve as an 

 osmotic membrane. Soak the membranes in 

 water for a few minutes to remove any 

 wrinkles. With ordinary narrow rubber bands 

 fasten the membranes tightly over the smooth 

 ends of two clean glass tubes with inside diam- 

 eters of 4r-7 mm. Sugar or salt solutions may 

 now be run into the tubes from the open ends, 

 using a wire or fine glass rod to direct the 

 fiow. The tubes should be filled to a height of 

 two or more inches and the level marked with 

 accuracy. Be careful to avoid bubbles. The 

 tubes may now be supported vertically by ring 

 stands with the bean testa in contact with 

 water in a glass dish. The height of the 

 liquid in the tube will rise almost immedi- 

 ately and will continue to do so for several 

 days. The usual variations of such experi- 

 ments as to the liquids used may be satisfac- 

 torily employed with this membrane. 



The writer has found this experiment the 

 most simple of the osmosis demonstrations to 

 set up. Five minutes is adequate with the 

 apparatus at hand. With ordinary care the 

 results are satisfactory in nine cases out of 

 ten. The students appreciate a real plant 

 membrane to illustrate plant osmosis. It is 

 advisable if time permits to set up the egg 

 experiment also for its general biologic value. 



As to the value of osmosis demonstrations in 

 elementary college courses in botany, we use 



