310 TEXTBOOK OF PLANT PHYSIOLOGY 



its development. In order to increase growth it is sufficient to 

 stimulate the germinating seeds. For this purpose, Popoff uses 

 3 per cent solutions of magnesium chloride and sulphate and man- 

 ganese sulphate and nitrate, as well as mixtures of these salts, and a 

 number of organic compounds, especially tannin and various nar- 

 cotic and antiseptic substances. The seeds are soaked in these 

 solutions for several hours and then sown in the usual way. 

 According to Popoff, as a result of such a treatment growth has 

 been accelerated, the size of the plants has increased, and consid- 

 erably larger crops have been obtained. In some of his experi- 

 ments this increase has amounted to as much as 50 to 100 per cent. 



Popoff 's experiments have attracted a great deal of interest, 

 since by stimulation of seeds a considerable increase in yield may 

 be obtained at little expense. But most investigators have not 

 been able to confirm his results, especially under field conditions. 

 Stimulation of seeds is a considerably more complicated affair 

 than it appeared at the outset. The conditions under which the 

 experiments succeed have not as yet been determined. 



In close relationship with the phenomenon of stimulation are 

 methods of forcing (Art. 87). In forcing, both chemical and 

 physical agents are applied as stimulants. As in the case of seeds, 

 so here the stimulus once given leads to changes in the character 

 of development of a plant which continue for some time. 



Under chemical stimulants are included also the so-called hor- 

 mones. They are formed by the organism itself, and produce 

 changes in the rate and direction of chemical reactions of the cells 

 and organs, thus altering the very characters of growth and devel- 

 opment of an organism. The action of hormones was first dis- 

 covered in animals in which they are formed by special ductless 

 glands, named glands of internal secretion. Such are the supra- 

 renal capsules, the thyroid gland, sexual glands, etc. The hor- 

 mone of the suprarenals, adrenalin, has been obtained in pure state 

 and its chemical composition has been established. Plant hor- 

 mones are more difficult to determine. The best studies of their 

 activity are in connection with increased division and the growth 

 of cells on the surface of healing wounds. 



It has been demonstrated by Haberlandt that the increased 

 division of cells results not from mechanical injuries nor an increased 

 access of air to the bared cells, but from altered chemical action of 

 the cut cells. Before disintegration they seem to release some sub- 



