288 TEXTBOOK OF PLANT PHYSIOLOGY 



flower plant. The same result may be obtained by using tips of 

 dandelions or flowering stems of poppies on coleoptiles of oats. 



The growth-promoting substance moves along the stem in a 

 definite direction, i.e., from the tip to the base, independent of how 

 the organ is oriented in space. According to Went, the mechan- 

 ism of its action consists in softening of the cell walls, which facil- 

 itates their stretching. It is possible that this substance is an 

 enzyme. The substance is used up in growth, the lower zone con- 

 taining less than the upper one. Went believes that the presence 

 of this substance determines the period of maximum growth. It 

 is interesting to note that the tip of a root secretes a substance 

 which slows down rather than increases the growth of the upper 

 zone of this organ. It appears, however, that the difference is not 

 due to the properties of the substance secreted but to the peculiari- 

 ties of the cells of roots in the zone of stretching. Experiments of 

 Cholodny show that a root tip increases the growth of decapitated 

 coleoptiles; and, on the contrary, the tips of coleoptiles hinder the 

 growth of decapitated roots. From this it follows that the growth- 

 promoting substance, or as Cholodny calls it, the growth hormone, 

 is evidently the same in all growing organs, but the various organs 

 react differently to its presence. 



At the present time, many experiments are being tried with 

 various substances to determine their ability to increase the growth 

 of decapitated vegetative organs. It may be noted that even saliva 

 causes an appreciable increase in the growth of oat coleoptiles and 

 stems of other plants. 



Elongation is followed by the third and last period of growth, 

 the stage of internal differentiation. The thin and stretched wall 

 now grows in thickness. The cell has ceased to increase in volume 

 and assumes its final form and size. The homogeneous zone of 

 growth gives rise to the various tissues of the mature organ, such 

 as the conducting and mechanical elements, etc. The internal 

 factors, which control differentiation, frequently accompanied by 

 the formation of special thickening in the cell walls, the loss of the 

 cell contents, etc., remain unknown at present. 



87. Periodicity of Growth. The Rest Period. Methods of 

 Forcing Plants. — Frequently all three stages of growth succeed 

 one another uninterruptedly. This results in even and continuous 

 growth. Such, for instance, is the growth of roots, in the longi- 

 tudinal section of which there may be found cells in all three stages 



