INTERRELATIONS BETWEEN PARTS OF PLANTS 401 



hyacinths, and other plants have also the capacity of producing 

 buds from leaf cuttings. 



Cuttings that are separated from the plant even when very 

 small in size may regenerate all the other parts of a plant. 

 Regeneration activity begins with the formation of callus on the 

 surface of the cut tissues. This results from an increased division 

 of cells adjoining the surface wound. Certain cells that had 

 completed their growth long before the injury was performed 

 may divide again under such conditions. 



Especially rapid is the formation of the callus in the immediate 

 vicinity of the phloem. This has 



given rise to the supposition that ^^ p^ ^wiL^ 



the phloem carries special hormones ^^ j^ j ^ 



that stimulate cell division. In the X-^^tV^T^-v^I Tp^ 

 case of injuries, the hormones enter \^ %?sM S^y . \^/ 

 the cells adjoining the phloem, ^i^l^-^^^^"^^ '0\ 

 inducing an increase of growth and ^^fc^X<t«?^^!5t=====^ 

 division. vfifl*' 



The callus consists primarily of >il 



undifferentiated parenchyma cells Fig. 123.— The formation of 



, . 1 , , 1 , • 1 j_ shoots on a Bryophyllum leaf {after 



m which the conductmg elements, Transeau). 

 the cambium and regions of grow^th, 



are later differentiated. 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 successful, it is important, therefore, that there 

 should be on the cutting at least one bud capable of developing 

 into a new shoot. In this connection, it should be noted that the 

 so-called ''dormant buds," which 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 morphologically lower end will 

 develop much faster and more extensively than at the upper end. 

 This cannot be attributed to a downward movement of organic 

 substances, as might appear probable at first sight; for if such 

 cutting is placed upside down, so that its morphological lower 

 end will be at the top, the callus will still be more pronounced 



