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THE AMERICAN NATURALIST [Vol. XLIV 



water and become the mono-hydrate as soon as the vapor 

 pressure reached 4.5 mm. of mercury. As soon as it 

 reaches 30 mm. the monohydrate takes up two more mole- 

 cules and becomes the trihydrate; while at 47 mm. and 

 above the trihydrate adds two more molecules and be- 

 comes the pentahydrate. Conversely, should the vapor 

 pressure fall below 47 mm., the pentahydrate loses water 

 and is transformed into the trihydrate, and so on. The 

 constitution of the copper sulfate, in fact, precisely re- 

 sponds to the vapor pressure of water around it. Many 

 like cases might be cited. The peculiarity of these is that 

 while the external factor may change steadily, the phys- 

 ical response will be discontinuous. Among the multi- 

 farious phenomena of irritability in plants are some 

 which are so nearly parallel to the behavior just de- 

 scribed as to suggest an analogous causation. 



It is possible that the application of the term irritabil- 

 ity to the behavior of crystals will seem an unwise strain- 

 ing of its usual sense. Irritability has often been 

 enumerated among the distinguishing properties of 

 protoplasm. Even Sachs wrote : 



Irritability is the irreat dist in-iiishin- characteristic of livinir organ- 

 isms; the dead organism is dead simply because it has lust its irritability. 



But if we are to consider irritability as a property— 

 a characteristic— of protoplasm, it must be understood 

 to be a conditional property, as strictly limited by cir- 

 cumstances as are the properties of non-living matter. 

 Steel may be highly tenacious at certain temperatures; 

 but at the temperature of liquid air it is as brittle as 

 glass and can be pulverized by a blow. A frond of 

 Laminaria when wet is a tough flexible strap, but when 

 dry it is so rigid and brittle that it can be broken to frag- 

 ments by slight bending. Many a plant is clearly irri- 

 table at moderate temperatures when well watered and 



conditions. Becquerel has dried seeds for six months in 

 a vacuum with barium hydroxid at 40°, sealed them for 



