Observations on Spirogyra. 423 



grm. The percentage of water, volatile and non-volatile 

 constituents, in Spirogyra is therefore as follows : — 



Water, . . . 96'S per cent. 



Volatile constituents, . 2'72 „ 



Non- volatile constituents, 048 „ 



100 per cent. 



This large amount of water led me to the following 

 experiment, showing to what extent turgor stretches the 

 elastic cell- wall. A slide is immersed in water in which are 

 a few threads, preferably of Spirogyra nitida ; one end of a 

 thread is fixed on the slide with the index finger and the 

 slide gently withdrawn, when the thread will lie in a straight 

 line. The slide and thread are arranged under a magnifying 

 power sufficiently high to allow the individual cells being 

 counted readily ; the ordinary eyepiece is then replaced by a 

 micrometer-eyepiece, and the number of cells covered by the 

 scale is counted and marked down ; then the same process 

 is repeated with another part of the same thread, to see 

 whether the number of cells counted the second time agrees 

 approximately with the first number, and if the difference 

 between the two numbers is only a slight one, i.e., not more 

 than half the length of one cell, the thread may be used for 

 the experiment. We apply now a 75 per cent, solution of 

 common salt, which will produce plasmolysis and cause the 

 thread to become shorter. The thread is arranged again, 

 and the number of cells covered by the scale on the eyepiece 

 counted. The average shortening is close on 10 per cent, of 

 the original length — e.g., in one case I counted 20 cells, 

 and after treatment with salt-solution 22 and part of a 23rd 

 cell. We may therefore say that the cells owe one-tenth 

 of their original size to turgescence. 



As Spirogyra is a plant very suitable for many observa- 

 tions and experiments, it is convenient to have always a 

 supply of fresh material at hand, and a few words on its 

 cultivation may not be out of place. When a mass of 

 Spirogyra material is placed in a vessel it will sink at first 

 to the bottom, then the more vigorous threads begin to grow 

 upwards towards the surface of the water, and in doing so 

 ribbon-shaped masses or strands will be formed : this 



