Jan., I92i] WIGGANS — OSMOTIC CONCENTRATIONS OF GUARD CELLS 
33 
Material 
By preliminary tests, it was found difficult to determine when epidermal 
cells were plasmolyzed if they did not contain pigment. This made it 
necessary to choose plants possessing pigmented cells in the lower epidermis. 
It was fairly easy to determine when guard cells were plasmolyzed on account 
of the density of their protoplasm. The plants found best for making 
quick and accurate determinations were Cyclamen, Iresine, Zebrina pendulay 
and young beet. The leaves of all these plants possess pigmented cells in 
the lower epidermis. The epidermis may be separated rather freely from 
the leaf parenchyma. This fact increases the rapidity with which mounts 
and determinations can be made. The work was begun in the laboratory 
of the Department of Plant Physiology at Cornell University at the sugges- 
tion of Dr. Lewis Knudson, and carried to completion in the laboratories at 
the University of Missouri. The same methods were followed at both 
places, and as far as possible the same kinds of plants were used in the tests. 
Results of Tests 
Zebrina pendula 
The first tests recorded on Zebrina pendula were made on November 27. 
The results are shown graphically in figure i. The first observation w^as 
l^A/M. ^AAi. jOAjn. 12. n\. Hp.ia. "/p.m 
Fig. I. Zebrina pendula. Observations made at Ithaca, N. Y. 
made at 9:30 a.m. At 10:30 a.m. the highest osmotic concentration of the 
guard cells was observed. From this hour to sundown there was a gradual 
decrease. On the other hand, the threshold concentration of the epidermal 
