Apr., 1923J 
ADDOMS — HYDROGEN ION AND PROTOPLASM 
215 
situations just what processes actually operate. Indeed, unless a substance 
is so colored that its presence can be actually seen in the protoplast, or unless 
its passage can be traced through the plant, the experimenter does not even 
know whether or not the substance entered the protoplast of which the 
structure is affected. In order to determine what process operates in a 
given situation, he must know just how the given substance affects the 
given protoplast. Very little of this kind of work has been done. 
The dark-field microscope offers the best method for this kind of study, 
because it makes possible observation of colloidal particles which are too 
small to be resolved by the ordinary microscope. The apparatus used in the 
present investigation consisted of a compound microscope fitted with a 
Zeiss cardioid condenser, a 1.8-mm. achromatic objective, and a 10 x 
ocular. The cardioid condenser requires intense illumination because the 
cone of light becomes very broad before entering the objective. A small 
arc lamp with carbons fixed at an angle of 70° was found satisfactory. The 
light was passed through an aqueous ammoniacal solution of copper sul¬ 
phate to remove the red rays, and was then focused on a plane mirror, from 
which it was reflected into the condenser. The slides and cover glasses 
were selected for a uniform thickness of 1.0 and 0.1 mm. respectively. 
They were cleaned with alcohol, dipped in collodion, dried, and then stored 
in water; immediately before a slide or a cover glass was used, it was wiped 
free from water and the thin film of collodion was stripped from the surface, 
thus removing all dust particles. 
The protoplasm of an actively growing root hair appears milky under 
the dark-field microscope because the particles are so small, so numerous, 
and so evenly distributed that the enlarging light cones overlap, thus making 
it impossible to distinguish the individual particles. In a very young root 
hair the protoplasm is dense and almost devoid of vacuoles. As the root 
hair grows, the protoplasm becomes less dense, vacuoles form and enlarge, 
and the cell is apparently at the height of its usefulness as an absorbing 
organ. The vacuoles continue to enlarge and begin to coalesce, and the 
protoplasm is crowded more and more toward the outside of the cell, so that 
finally it is but a thin film separating the cell sap from the cell wall, and 
the root hair is of little value to the plant. To study the effects of nutrient 
solutions on the protoplasm of root hairs, it is thus obviously necessary to 
select cells that are in the second stage of their grand period of growth, for 
at this age their condition determines their value to the plant. 
Root hairs of wheat grown in the nutrient solutions described above 
differ markedly in the appearance of their protoplasm, when they are exam¬ 
ined at the age of approximately maximum usefulness to the plant. These 
differences are shown in figure 1, Plate XXIII. In hairs from plants grown 
in solution 1 (pH 3.94) the protoplasm is evenly distributed through the 
cell and shows no suggestion of precipitation 4 or of aggregation into masses. 
4 To avoid ambiguity, it becomes necessary to define the following terms, which have 
been used in different senses by different colloid chemists: 
