NUTRIENT SALTS. 73 
attracted is direct; but if the cell in which the material is used up is separated 
from the substratum by intervening cells, the attraction must act through all those 
cells upon it. The substance consumed must be’ taken in the first place from the 
cell adjoining the consuming cell on the side towards the periphery; this cell again 
must take it from its neighbour, which is still nearer the periphery, and so on 
until the external cells themselves exercise their influence upon the nutrient sub- 
stratum. Thus, one may regard the growing cells in which substances are used 
up, as centres of attraction with respect to those substances. This also explains 
why it is that the influx of food-salts takes place only so long as the plant is grow- 
ing; and we see, too, that the direction of the current must vary according to the 
position of the growing cells, and according to the degree of their constructive 
activity. 
But that one plant prefers one substance and another another—that one species 
attracts iodine, a second sodium, and a third iron—can only be interpreted as a 
result of the specific constitution of the protoplasm. The protoplasm of a growing 
cell which contains no iodine does not require that substance either, for the pro- 
cesses of transmutation and storage. A protoplast of this kind will not therefore 
be a centre of attraction for iodine, but will draw from the environment with 
great force substances which are its essential constituents. Having gained this 
conception of the absorption and selection of food-salts, we are able to imagine 
the possibility of a substance being sought after by one species whilst acting as 
poison on another. Iodine itself exercises a prejudicial effect on many plants, 
even when present in very small quantities. Cell-membranes in immediate contact 
with a medium containing iodine are modified as regards their structure by the 
iodine: their pores are enlarged, lose their value as orifices adapted to the admit- 
tance of certain food-salts in limited quantities, and they no longer prevent the 
influx of injurious substances. Ultimately they die, and by so doing the entire 
plant suffers. On the other hand, plants to which iodine is an indispensable 
constituent are not hurt in any way by the presence of small quantities of this 
substance in the nutrient medium: their cell-membranes are neither paralysed 
nor destroyed, and suction is able to take place through them in a perfectly normal 
manner. But we must in this case specially emphasize the condition of the amount 
being small, for a larger quantity of this substance is positively injurious even to 
plants which require iodine. 
The general rule for a great number of plants is that they thrive best when the 
food-salts necessary to them are supplied in very dilute solutions. An increase in 
the quantity of the salts administered not only fails to promote development, but, 
on the contrary, arrests it. This is the result even if the salts are such as are 
absolutely necessary in small quantities to the plants in question. A very minute 
amount of an iron salt is indispensable to all green plants; but, if a certain 
measure is exceeded, iron salts have a destructive effect on the cell-membranes and 
protoplasm, and cause the plant to die. But at what point the boundary lies 
between salubrious effects and the reverse, where the beneficial action of particular 
