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 eflect 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 sufiers. 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 



