GROWTH 287 



tissues on which they develop, although the absolute degree of 

 their osmotic pressure is not great. 



The growth of cells in the zone of elongation is closely related 

 to the activity of the region of embryonic tissue which lies above it. 

 If the growing point of a stem is cut, then the elongation of the 

 lower zone immediately decreases and is accelerated only after the 

 process of regeneration starts at the place where the cut was made. 

 This slowing down of growth is especially conspicuous on flowering 

 stems when the flower buds are removed, and also on coleoptiles of 

 cereal plants that are just breaking through the soil. If the sev- 

 ered growing point is attached again by means of a drop of water 

 or gelatin, then growth is notably increased. Recent investigations 

 by Went (1927) show that the growing point of a coleoptile 

 manufactures a special substance, called by him a "growth-pro- 

 moting substance," which descends in the growing organ and 

 causes stretching of its cells. When the severed growing point is 

 placed with its cut surface on a thin layer of agar gelatin and left 

 for about 2 hr., the layer of agar absorbs this growth-promoting 

 substance. If the growing point is removed from the gelatin and 

 a piece of the latter is placed on the cut surface of the coleoptile, 

 then growth immediately increases. Wendt has even determined 

 the rate of diffusion of this growth-promoting substance in agar 

 gelatin and also its approximate molecular weight, which was in the 

 neighborhood of 350, or approximately equal to the molecular 

 weight of sucrose. Further than this the chemical nature of the 

 substance is not known. 



This growth-promoting substance can be formed not only in 

 growing tips but also in other parts of vegetative organs. According 

 to Cholodny, it is formed in the phloem of the hypocotyl of the 

 lupine and sunflower. If, by means of a special drill, the central 

 cylinder containing the phloem elements of a section of the hypo- 

 cotyl is removed, the rate of growth of the section is noticeably 

 decreased. If now a tip cut from the growing point of the coleop- 

 tile of corn is placed into this hollow stem, the growth of the hypo- 

 cotyl will increase considerably, since it is under the influence of 

 the growth-promoting substance secreted by the tip of the corn 

 coleoptile. 



From these experiments one may conclude that the growth- 

 promoting substance is not specific, as the secretion of the tip of 

 coleoptile of corn also increases the rate of growth of a lupine or sun- 



