REPRODUCTION 361 



wound. Even cells that had completed their growth long before 

 the injury was performed may divide again under such conditions. 

 Especially energetic is the formation of the callus in the immediate 

 vicinity of the phloem. This has given rise to the supposition 

 that the phloem carries special hormones which stimulate cell 

 division. In the case of injuries, the hormones enter the cells 

 adjoining the phloem, inducing an increase of growth and division. 



The callus consists primarily of undifferentiated parenchyma 

 cells in which the conducting elements, the cambium and regions 

 of growth, are differentiated. The latter produce the adventitious 

 roots. Numerous roots are developed also from the cambium of 

 the cutting, but new shoots are formed with great difficulty from 

 the callus. In order that propagation by cutting may be suc- 

 cessful, it is important, therefore, that there should be on the cut- 

 ting at least one bud capable of developing into a new shoot. In 

 this connection, it should be noted that the so-called dormant buds, 

 that normally would not have developed for many years, if at all, 

 will produce shoots on a cutting. 



If a small section is cut from the middle of a branch having two 

 injured surfaces, one at the top and the other at the bottom of the 

 cutting, the callus at the lower end will develop much faster and 

 more extensively than that at the upper end. This, however, 

 cannot be attributed to a downward movement of organic sub- 

 stances, as might appear probable at first sight, for if such a cut- 

 ting is placed upside down, so that its morphological lower end will 

 be at the top, then the callus will be still more pronounced at this 

 end, though the force of gravity will act under these circumstances 

 in the opposite direction (Fig. 150). When sections of axial organs 

 react differently at their two morphologically opposite ends, the 

 phenomenon is known as "polarity." 



Polarity manifests itself clearly in the distribution of roots and 

 the development of buds of a cutting. If a cutting is suspended 

 in normal position in a humid atmosphere, the roots will be formed 

 largely at its lower end, and the shoots will be produced from buds 

 situated at its upper end; but when the shoot is suspended in an 

 inverted position, the roots will nevertheless arise from the 

 morphologically lower end, though it will now be at the top, while 

 the shoots will develop at the morphologically upper end, now at 

 bottom. If. the center of the cutting is girdled, the effect will be 

 the same as that of the division of the cutting into two separate 



