148 



THE CELL AND PROTOPLASM 



pea depends upon the top. "We can 

 imagine any other number of combinations 

 of substances which cannot be synthesized 

 by root cells, and future work will show 

 what variations on this general theme are 

 found in nature. But as far as informa- 

 tion is available, we can say that vitamin 

 Bi is a general root growth hormone in 

 the higher plants. It is produced in the 

 leaves in light. 



Root growth provides us with the in- 

 formation necessary to trace the phylogeny 

 of the hormones, because it will clarify a 

 number of puzzling facts which we will 

 encounter later. Consider a self-sufficient 

 or autotrophic primitive acellular plant, 

 such as a bacterium. With available 

 energy it is able to build up its food, 

 which will serve as a source of energy, or 

 according to van Niel (1940), as a source 

 of break-down units for its metabolism and 

 growth. For our purposes we can disre- 

 gard the difficulties involved in the syn- 

 thesis of the complex proteins, nuclei-pro- 

 teids, etc., which are essential for its 

 growth. But as building units for these 

 proteins, enzymes, etc., a number of molec- 

 ular nuclei, such as benzole, pyridine, 

 pyrimidine, purine, imidazole, thiazole, and 

 sterole, must be available. These cannot 

 be produced by simple dehydrogenations 

 or dehydrations, the most common chem- 

 ical reactions occurring in living cells. 

 Whereas the partly heterotrophic organ- 

 isms can derive some of these building 

 stones from their surroundings, the com- 

 pletely autotrophic cell is compelled to 

 synthesize them from very simple ma- 

 terials. As soon as a cell has lost the 

 ability to make one of these building stones, 

 it becomes dependent upon other cells. In 

 this way a basis for interdependence of 

 cells or organisms is laid. 



Hormones and vitamins are essential 

 building stones of cells, or they have at 

 least definite functions in cells, although 

 cells are unable to produce them. In the 

 case of hormones other cells in the organ- 

 ism have not lost the ability to form them ; 

 hence an absolute dependence of the using 

 cells upon the producing cells develops. 



If for other substances the reverse rela- 

 tionship exists, then the two groups of cells 

 are absolutely interdependent. The first 

 cannot grow without a certain activity of 

 the second, but again that second will be 

 limited by the first. Such a simple prin- 

 ciple underlies the unity of an organism. 

 How simple and how flexible it is has also 

 been stressed by Szent-Gyorgyi, and is 

 demonstrated by the fact that for rats and 

 plants ascorbic acid is not a vitamin, be- 

 cause they synthesize it themselves, but for 

 other animals it is a vitamin. Thiamin is 

 a vitamin for animals, a hormone for most 

 plants, and a vitamin for some other plants 

 (Bonner 1937a). Nicotinic acid is a hor- 

 mone in some plants, a vitamin for ani- 

 mals. All of these substances are rather 

 simple building stones for the cell, just as 

 flavine, adenine, etc. Therefore we may 

 look forward to the discovery of many 

 more simple molecular nuclei as hormones 

 or vitamins, and to discovering in which 

 specific reactions known hormones take a 

 part. 



In discussing the correlations in the 

 above-ground parts of plants, it is desir- 

 able to go back historically. Sachs (1880, 

 1882) gave the first logical analysis of the 

 phenomenon of correlation. He based this 

 analysis on the following considerations. 

 In the course of development there is an 

 increased differentiation of cells. When 

 different organs develop from cells out of 

 homogeneous tissue, the principle of causal- 

 ity implies that at some time the initial 

 cells of these organs differed in some re- 

 spects. Since at the outset they were sup- 

 posed to be equal, this difference must have 

 developed later. Further consideration of 

 teratological cases and galls led Sachs to 

 the conclusion that minute amounts of 

 certain chemicals, so-called organ-forming 

 substances, were responsible for this de- 

 velopment of difference. He supposed that 

 these substances were transported polarly 

 in the plant, and thus the polar organ 

 formation, according to Sachs, was due 

 to the polar movement of organ-forming 

 substances. 



In the subsequent 30 years a number 



