INTERRELATIONS BETWEEN PARTS OF PLANTS 403 



numbers above the ring cut (Fig. 125). On the control cutting, 

 the greatest number of roots will develop at the extreme end. 

 Of course, the impeding of the flow of nutritive substance by the 

 ring cut plays a certain role, but a greater influence is exerted 

 by the retention of the root hormone. 



Recently the Belgian physiologist Bouillenne and the Dutch 

 physiologist Went demonstrated quite definitely the existence 

 of a special root-forming hormone, which they 

 have termed '' rhizocaline." UnUke the growth- 

 promoting hormone, auxin, which was men- 

 tioned in Art. 29 and which is elaborated in the 

 growing tip of the stem, rhizocaline is produced 

 by leaves during photosynthesis. This explains 

 the fact, long ago observed, that the remainder 

 of at least one leaf on the cutting is of great sig- 

 nificance for varieties that root with difficulty 

 and that in such cases rooting proceeds more 

 readily in the fight. The base of the cutting, 

 i.e., the place where the roots are formed, must 

 be shaded, however. Rooting is considerably 

 stimulated by girdling or ringing of the cutting 

 before its removal from the mother plant 

 (Eremeev, 1933). This stops the outflow of 

 rhizocaline and thus contributes to the initia- 

 tion of roots above the girdle. After cutting 

 and covering the base of the cutting with soil, ior n- 



^ . ,, biG. 125. — ijrir- 



these root initiations develop rapidly. aied willow stem 



93. Grafts and Chimeras.— If a cutting of a with roots growing 



. primarily above the 



plant is not allowed to root but is set upon gj^^ie. 

 another plant, with which it is capable of uniting 

 by growth, it wifi begin to develop ^nd eventually produce a 

 new plant. But the cutting under such conditions will not 

 form its own root system. This operation is known as ''graft- 

 ing." The transplanted cutting is called the ''scion"; and 

 the plant with which it is united, the "stock." Grafting is a 

 method of propagation common in horticulture, as the properties 

 of improved varieties are not inherited through reproduction by 

 seeds. Moreover, the cuttings of many fruit trees do not root 

 readily. Seedlings obtained from the seeds of wild fruit trees 

 commonly serve as stocks for this purpose. They are usually 



