STOMATA DURING WILTING. 8 1 



CLOSURE OF STOMATA DURING WILTING. 



Francis Darwin (1898, p. 543) inferred from a series of experiments upon a 

 dozen or more plants that, upon the removal and consequent withering of 

 the leaf, there occurs during the first few minutes (from 3 to 10, according 

 to his data) a temporary opening of the stomata, followed by the gradual 

 closure ordinarily expected.* It is obvious that, if such a temporary opening 

 occurs within the first few minutes of withering, it might easily have escaped 

 attention, both on account of the difficulty of recognizing the early stages 

 of wilting, and especially because of the difficulty of determining exactly 

 what the stomata do without seeing them. Darwin's method was to apply 

 the horn hygroscope to the leaf just removed and record the readings. The 

 initial readings may well be imagined to have been caused by an increased 

 outpour of water-vapor caused by an initial reduction of tensions, and not by 

 the increase in the size of the stomatal openings. That this may indeed have 

 been the case, I am led to believe by the results obtained by me with a 

 simple apparatus (pi. 5, figs. 1 and 2) designed to give some idea, approaching 

 exactness, of what occurs during wilting as regards the leaf as a whole, and 

 what the rate of wilting for a particular leaf may be. This consisted of a suit- 

 able stand, with an arrangement of pins and corks, together with a graduated 

 index, so that a leaf, having just been removed from the plant, might be 

 oriented in such a way that the movements during wilting could be recorded 

 by watching the deflection of a bristle or glass-thread indicator, previously 

 fastened to the apex of the leaf by means of shellac. The previous constant 

 position of the leaf was insured by watching the indicator for some time after 

 its attachment and while the leaf was not yet removed from the plant. 



By studying the process of wilting in Verbena by means of this apparatus, 



I found that in leaves of fair or full maturity the period of wilting embraced 



two distinct phases. Thus, if a leaf, with the ventral surface up, be so oriented 



that the indicator is placed in a horizontal position, this will at first move 



upwards for some time, after which it will begin to drop. This is illustrated 



by the following readings at 1 -minute intervals, the plus sign indicating 



upward and the minus sign downward movement. 



+ — 



o, 1, 1.5. 3. 4-5. 6, 7. 7. 6.5, 5-5, 4. 2, o, 2.5, 5, 8, 10.5, 12, 14. 



The total length of the whole period and the ratio between the two phases 

 will depend upon the maturity of the leaf and the consequent interactions of 

 the mechanical tissues present. In quite young leaves there is no upward 

 movement at all. Not all kinds of leaves show an upward movement upon 

 wilting, though I have noted it in the so-called sensitive fern Onoclea sensi- 



♦Darwin records experiments showing this general wilting and stomatal closure (loc. cit.) . 



