306 Mr Darwin, Observations on Stomata [May 10, 



It is impossible to make two instruments identical, so that 

 the readings of one instrument are not strictly comparable with 

 another. The chief use of the hygroscope, and for this it is 

 admirably suited, is to compare the leaves of an experimental 

 plant with those of a control, or to observe sudden or gradual 

 changes of reading on a given leaf. 



II. 



In a forthcoming paper I shall give a full account of the 

 results which I have obtained. In the present publication I give 

 a brief statement of the facts which may be easily demonstrated 

 with the hygroscope, together with some results which are believed 

 to be new. 



1. The hygroscope index remains at zero on the upper surface 

 of a hypostomatal leaf, indicating that there is a low degree of 

 transpiration from that surface. 



2. There is active transpiration on the lower surface of a 

 floating leaf (e.g. Nymphaea) when exposed to air. 



3. In a terrestrial leaf with stomata on both surfaces, the 

 behaviour of the stomata is different on the two surfaces. Thus 

 the upper stomata are more sensitive to external conditions and 

 are frequently shut when the lower ones are opened. 



4. The typical stoma closes (either wholly or partially) in 

 darkness. The process is not a sudden one, the readings of the 

 hygroscope fall gradually to zero. It is difficult to generalise as to 

 the time which closure in darkness or opening in light requires ; 

 half-an-hour would represent fairly rapid effect. 



5. The natural diurnal course of the stomata can be easily 

 followed by taking readings throughout the day and the behaviour 

 of the stomata thus graphically represented. The rise in the 

 early morning and the fall at night are both rapid, the inter- 

 mediate region of the curve may be either more or less flat, or it 

 may present a continuous rise and fall. 



6. A certain degree of closure at night is almost universal in 

 typical terrestrial plants and is generally absent in water-plants 

 and marsh-plants*. Since the hygroscope does not distinguish 

 between partial and complete closure, these results are not neces- 

 sarily at variance with those of Leitgeb-f. 



7. The nocturnal closure is a periodic phenomenon: this is 

 not so strongly marked as the nyctitropic movements of leaves, 

 since the stomata of a leaf placed in a dark room at night are 

 not found to be open in the morning. 



* Caltha and Trianea however close at night. 



t Mittheilungen aus dem Bot. Institut zu Graz, 1886. 



