Plant Qrowth'Suhstances 



Professor Boysen Jensen, on pages 115-6 of his book on 

 growth-substances, has said: "Numerous experiments have 

 shown that without growth-substances, growth of the shoots 

 of higher plants cannot take place. . . . The increase in cell- 

 wall boundaries is only one manifestation of the fundamental 

 ability of an organism to build itself out of the materials of its 

 environment." 



As Boysen Jensen reminds us, there are several modes or 

 kinds of growth. Growth in length or height may be due to 

 the lengthening of a fixed number of cells, without new cells 

 being formed: that is rather like a stretching, only, the force 

 comes from inside, not outside. Growth in length or height 

 may also take place by an increase in number of cells, so that 

 the growing-point is pushed out to make more room for the 

 older tissue. There is also a mode of growth which starts the 

 formation of new sorts of tissue, as when a leaf-bud begins to 

 form. This last kind of growth is due to cell-differentiation. 



The effects of growth-substances that have been most 

 intimately studied are those that arise from the first mode, 

 namely, the increase in length of a fixed number of cells. 

 Young seedling oats have become a favourite material for 

 demonstrating the existence of this type of growth. This is 

 partly because it is relatively simple to see what is happening 

 when a certain part (the coleoptile, or first leaf-sheath) of a 

 young seedling oat-plant that has had its tip cut off, has been 

 treated with a trace of the substance that is being investigated. 

 The substance is applied to one side of the cut surface of the 

 stump (the tip, which contains the plant's own shoot-growth 

 hormone, is thrown away). The applied substance causes the 

 cells on the treated side to grow longer (provided, of course, 

 that it is active as a growth-substance). The result is that 

 the leaf-sheath bends to one side in order to relieve the strain 

 caused by the unequal growth. The bending is easily visible. 



This is the essence of the oat-test. Oat seedlings are em- 

 ployed because they are cheap, easily grown and handled, 

 are responsive, and otherwise convenient. 



The effect of light in causing bending of stems is known to 



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