6^6 LECTURE XXXVII. 



only to be obtained in the original home of these plants by careful observation of 

 their relations with the external world. Of course the irritability of the foliage leaves 

 in the cases indicated can only be of use if they are sensitive in a high degree ; it 

 would be very difficult to detect any such use in the slightly irritable foliage leaves of 

 our Wood Sorrel, Robinia, and several other plants, which only fold up slowly when 

 sharply struck or shaken. 



