27 
While all recent writers on the subject have assumed that changes in 
turgor are responsible for the changes in turgescence with variations in 
the illumination, it has never, so far as I am aware, been demonstrated 
that the turgor really does vary.‘7 I myself have tried to measure such 
a change with divers plants, but without success. However, with the 
coconut it is easy to determine that the turgor is much higher in light 
than in darkness, though the actual differences are rather inconstant. 
The turgor of a single pair of guard cells can be demonstrated to change 
during a prolonged experiment; but as this involves plasmolysing the 
pair at least twice, and, as a rule, subjecting it to several strong plasmo- 
lysing solutions, each of which must be given time to act, the cells are 
likely to suffer changes from this treatment. The evidence taken from 
the observation of many different stomata, in their natural condition, at 
different times of day, is more valuable. 
The turgor on sunny afternoons is usually about equal to normal potassium 
nitrate. Sometimes it exceeds even this high figure. Thus at 3.30 p. m. Novem- 
ber 25 these measurements, in microns, were made: 
| 0.6 nor- 0.7 nor- | 1nor- | 
| mal. | mal. | mal. | 
| 
aa SAS yore betes ea 
o 
| Pre Witth 2) ss | 2.§ 1.5 | er 
While the particular stoma under observation was not measurably open, about 
one in five on the section was open to the extent of at least 1 », the plasmolysis 
of any guard cells being very doubtful; but when the normal potassium nitrate 
was replaced by glycerin, plasmolysis was evident everywhere, and all stomata 
were closed. If they were examined early in the morning the guard cells were 
usually found to have their turgor equal to somewhat less than 0.7 normal potas- 
sium nitrate but rarely below 0.6 normal. In direct sunlight the increase is an 
immediate one. 
The action of prolonged darkness is very different from that of the 
mere nocturnal lack of light. <A leaflet was kept in darkness, inside a 
wooden cylinder, for ten weeks, at the end of which time its turgor, as 
compared with that of a neighboring pinna under ordinary conditions, 
was: 
From nor-, 
| From mal leat- 
darkness. let (in 
morning). 
Name. 
Epidermis ____- ee | 0.5 0,25-0.3 
Goard Cells. 52 0.9-1.0| .6-.7 
| Parenchyma’ —.._--.-_=.--- | 20 | tik Se | 
‘The term “turgor” is used to express the osmotic pressures of the internal 
fluid of a cell. On the other hand, the expression “turgescence” applies to the 
strain resulting from the interaction of the force of the osmotic pressure (the 
diffusion tension of the solute) on the one hand and that of the resilience of the 
cellulose membrane on the other. Copeland, Ann. of Bot. (1902), 16, 330. 
