2 Introduction 



whole subject leads one to conclude that in general both lavourable and 

 unfavourable conditions of nutrition are present side by side, and only 

 when a balance is struck in favour of the good conditions can satis- 

 factory growth take place. As indicated above, experiments have shown 

 that the very substances that are essential for plant food may be, in 

 reality, poisonous in their action, exercising a decidedly depressing or 

 toxic influence on the plant when they are presented singly to the roots. 

 This toxic action of food salts is decreased when they are mixed 

 together, so that the addition of one toxic food solution to another 

 produces a mixture which is less toxic than either of its constituents. 

 Consequently a balanced solution can be made in which the toxic effects 

 of the various foods for a particular plant are reduced to a minimum, 

 enabling optimum growth to take place. Such a mixture of plant foods 

 occurs in the soil, the composition of course varying with the soil. 



While the earliest observations set forth the poisonous action of 

 various substances upon plants, it was not long before investigators 

 found that under certain conditions these very substances seemed to 

 exert a beneficial rather than an injurious action. The poisons were 

 therefore said to act as " stimulants " when they were presented to the 

 plant in suflBciently great dilution. This stimulation was noticed with 

 various plants and with several poisons, and a hypothesis was brought 

 forward that attempted to reconcile the new facts with the old con- 

 ceptions. Any poison, it was suggested, might act as a stimulant, if 

 given in sufficiently small doses. It will be seen in the following pages 

 that this is not universally true, such substances as copper, zinc, and 

 arsenic failing to stimulate certain plants even in the most minute 

 quantities so far tested. 



Of recent years investigators in animal physiology have brought 

 into prominence the striking effect of minute quantities of certain 

 substances in animal nutrition, as for example iodine in the thyroid 

 gland (see E. Baumann, 1895). This and other work has rendered it 

 imperative to re-examine the parallel problems in plant physiology. 



The words "stimulant" and "stimulation" themselves need more 

 precise definition. As a matter of fact the "stimulation" noticed by one 

 observer is not necessarily held to be such by another. Stimulation 

 may express itself in various ways — the green weight and the general 

 appearance of the fresh plant may be improved, the dry weight may 

 be increased, the transpiration current may be hurried up, entailing 

 increased absorption of water and food substances by the roots 

 assimilation processes may be encouraged. But these benefits are 



