Introduction 3 



not of necessity correlated with one another, e.g. a plant treated with 

 a dilute solution of poison may look much healthier and weigh far 

 more in the green state than an untreated plant, whereas the latter 

 may prove the heavier in the dry state. To a market gardener to 

 whom size and appearance is so important, stimulation means an 

 improvement in his cabbages and lettuces in the green state, even 

 though the increased weight is chiefly due to additional water absorbed 

 under the encouragement of the stimulative agent, whereas to a 

 scientific observer, the dry weight may give a more accurate estimate 

 of stimulation in that it expresses more fully an increased activity in 

 the vital functions of the plant whereby the nutritive and assimilative 

 processes have gone on more rapidly, with a consequent increase in 

 the deposition of tissue. 



While stimulation expresses itself in the ways detailed above 

 poisoning action also makes itself visible to the eye. Badly poisoned 

 plants either fail to grow at all or else make very little or weak growth. 

 Even when less badly affected the toxic action is well shown in some 

 cases by the flaccidity of the roots, and in others by the formation of 

 a "strangulation" near the crown of the root, which spreads to the 

 stem, making it into a thin thread, while the leaves usually wither and 

 die. If such plants as peas are able to make any shoot growth at all 

 the roots show signs of a desperate attempt to put forth laterals. The 

 primary root gets much thickened and then bursts down four sides, 

 the tips of the laterals all trying to force their way through in a bunch, 

 but failing to do so on coming in contact with the poison. Most curious 

 malformations of the root arise from this strong effort of the plant to 

 fight against adverse circumstances. 



While all the inorganic substances examined in this monograph are 

 toxic in high concentrations, some lead to increased growth in lower 

 concentrations, while others apparently have no effect. In this sense 

 all substances could be classed as toxins, even the nutrients. Thus the 

 old distinction between toxin and nutrient has now lost its sharpness, 

 but it does not lose all its significance. The old "nutrients" had 

 certain definite characters in common, in that they were essential to 

 plant growth, the growth being in a great degree proportional to 

 the supply, a relatively large amount of the nutrients being not only 

 tolerated but necessary. The substances dealt with more particularly 

 in this book have none of these characters. Even those that cause 

 increased growth do not appear to be essential, at any rate not in 

 the quantities that potassium, phosphorus, nitrogen, &c., are essential, 



12 



