756 
Journal of Agricultural Research 
Vol. XXIV, No. 9 
was grown in Hoagland’s nutrient solution in which the calcium had 
been replaced by potassium; 500 cc. of this solution was evaporated 
to a small volume and was then tested and found to be calcium-free. 
Within 3 days after the experiment was begun, the root tips of the seed¬ 
lings in the calcium-free cultures were becoming brown and failed to 
show any evidence of growth. After 7 to 10 days the root tips of most 
of the seedlings were becoming slimy and gelatinous. At the end of 2 
weeks most of the roots appeared to be dead. The full amount of cal¬ 
cium of Hoagland’s nutrient solution was then added in the form of 
calcium nitrate to six of the jars. Within a few days many of the seed¬ 
lings began to recover, although several of the seedlings in advanced 
stages of decomposition failed to respond to the addition of the calcium 
nitrate. 
In Plate i, A, B, are shown various stages in the progressive killing of 
the primary root, beginning with the root tip and proceeding toward the 
cotyledons. Plate i, B, shows some of the roots of Plate i. A, somewhat 
enlarged. (The material closely held by the roots is composed of small 
pieces of sphagnum from the germination box.) It will be seen that 
when calcium was added to the solution, laterals were developed, the 
lowermost lateral always sharply separating the living from the dead 
portions of the primary root. Frequently in the absence of calcium the 
primary root was unable to grow. In such cases, when calcium was 
added, a lateral rootlet was produced immediately back of the root 
tip and this lateral then assumed the position of the primary root (Plate 
I, A, B). One of the most characteristic effects of a lack of calcium 
is the gelatinization of the surface layers of the root. This process is 
most active in the apical region of the root and progressively decreases 
in intensity toward the upper portion. As time goes on the gelatinization 
proceeds inward until finally the root becomes translucent. 
In the initial stages of this process of gelatinization, recovery is possible 
if a suitable amount of a calcium salt is added. The root then regains 
its firm white appearance and functions properly. In advanced stages 
of gelatinization recovery does not follow the addition of a calcium salt, 
although new laterals may be produced from the upper portion of the root. 
Herbst (2), Hansteen Cranner (J), and others have observed a similar 
gelatinization in other organisms in the absence of calcium and likewise 
have observed recovery when calcium salts were added. 
There is thus evidence that the stunted appearance of roots in pure 
NaCl solutions above discussed is not due to the toxicity of NaCl but 
to the lack of calcium. Further study of this factor was made in which 
more concentrated solutions of NaCl, having toxic properties, were used. 
Seedlings placed in Hoagland’s solution minus calcium failed to develop 
although it contained but 30 parts per million NaCl. They speedily 
recovered and grew when CaClj was added. Another series in calcium- 
free Hoagland's solution plus 1,000 parts per million NaCl likewise 
failed to develop. When CaClj was added to this latter solution the 
plants made good growth in spite of the amount of NaCl present. 
It appears, therrfore, that ^e harmful effect of 100 Na:o Ca in dilute 
solutions is not due to the lack of ions which antagonize the sodium, but 
rather to calcium starvation, and that the result is conspicuous in the 
case of a plant like citrus which is extremely sensitive to calcium. In 
solutions containing only 0.004 M. NaCl, it seems improper to speak of 
a toxic action upon the roots. (Cf. True, 8 i) 
