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TEXTBOOK OF PLANT PHYSIOLOGY 



parts. This leads to the conclusion that polarity is most likely 

 caused by the accumulation of some substances which move in the 

 bark in a definite direction and which stimulate at the point of 

 their concentration the development of both the callus and the 

 roots. Sachs has named these "root-forming substances," by 

 analogy to the flower-forming substances, which have been dis- 

 cussed (Art. 101). Some recent investigators have considered 



these to be hormones. According 

 to Loeb, the root-forming hormones 

 originate in the leaves, whence they 

 descend through the stem, while sub- 

 stances which delay the growth of 

 roots ascend through the stem. As 

 long as a branch is attached to the 

 tree, no formation of roots takes 

 place; but, if the vessels through 

 which the downflow of these hormo- 

 nes proceeds, are intersected, then 

 the latter will accumulate at the 

 surface of the cut and cause the 

 formation of roots. It has not been 

 possible, however, to isolate these 

 hormones and produce with their aid 



Fig. 150. — Poplar cuttings sus- the formation of roots. 



pended in humid atmosphere. On 105. Budding and Grafting. 



the left, in norma! position. On *>«». & uu« v^ ± «.* n" 6 . 



the right, in a reversed position, Chimeras. — If a cutting of a plant 



showing polarity in callus forma- • . 11 j , , i . ■ , 



tion (after Moiisch). 1S not allowed to root, but is set 



upon another plant, with which it 

 is capable of uniting by growth, it will begin to develop and 

 eventually produce a new organism. But the cutting under 

 such conditions will not form its own root system. This operation 

 is known as "grafting." The transplanted cutting is called the 

 scion, and the plant with which it is united, the stock. Grafting 

 is a common method of propagation in horticulture, as the proper- 

 ties of improved varieties are not inherited through reproduction 

 by seeds, while the cuttings of many fruit trees do not root readily. 

 Seedlings obtained from the seeds of wild apples or pears com- 

 monly serve as stocks for this purpose. They are usually cut off 

 at the crown and a small cutting of an improved variety is then 

 grafted upon the remaining stump. 



