CELLULAR ANATOMY OF THE ROOT 229 



also it is profitable to try a simple demonstration experi- 

 ment proving that the growth of the root is entirely at the 

 tip, which can be done by taking a large root of a ger- 

 minating Bean and marking it with waterproof India ink 

 at short regular intervals ; it is then allowed to grow on 

 farther in a thistle tube, as described in works on physi- 

 ology. The marks may be put on with a stretched thread 

 dipped in the ink ; such marks will not run as do those 

 made with a pen or brush. It is particularly important 

 that students observe that the roots contain no openings, but 

 are a closed system. The distinctness of the growing point, 

 and the protective cap, should be noted. 



The explanation of osmosis, the physical process by which 

 the liquids are absorbed in both cases in Experiment 9, is 

 not easy for beginners, nor are botanists and physicists agreed 

 upon the precise nature of the process. It will probably 

 have to be sufficient with beginners to point out to them 

 the physical fact, illustrated fully by Experiment 9, that when 

 a certain solution is on one side of a membrane wettable by 

 water, and water on the other, the water will pass in, while 

 if the membrane is wettable by the solution, some of it will 

 pass out, though not so much as enters ; but if the mem- 

 brane is not wettable by the solution, none of the solution 

 will pass out. For the teacher's own satisfaction, however, 

 the subject should be well worked out, whether he gives it 

 to his students or not. The membranes have no holes that 

 the most powerful microscope can discover, yet there must 

 be openings of some kind, as otherwise the water could not 

 pass. These are supposed to be spaces between the ulti- 

 mate particles, called " micellae," of which the membrane is 

 believed to be composed. There are two possible views as 

 to the nature of the osmosis, one, that water is strongly 



