A FRUITING ORANGE THORN 



Thorns Are Modified Leaves or Branches. On Orange Trees They Are Nuisances. 

 Thornless Variety May Be Produced 



A, D. SlIAMEL AXD C. S. POMEROV 



THE Century Dictionary defines a 

 thorn as ''a sharp excrescence on 

 a plant : usually a branch or the 

 termination of a stem or branch, 

 indurated, leafless, and attentuated to 

 a point." Botanically speaking, thorns 

 arc modified leaves or branches. 



A striking illustration of the rela- 

 tionship of some thorns to branches 

 is shown in the accompanying figures. 

 These show the development of abnor- 

 mally large thorns on rapidly growing 

 branches of the Washington Navel 

 Orange {Citrus siiicnsis (L.) Osbeck). 

 Some of the thorns have developed into 

 branches bearing fruit, leaves and sec- 

 ondary thorns. These varying stages 

 of development illustrate the fact that, 

 in the citrus, thorns are truly modified 

 branches. (Figs. 8 and 9.) 



THORN BEARS AN ORANGE 



As can be seen in the illustration, one 

 of the thorns bears a small orange. 

 The young navel orange is borne at 

 the tip of the thorn which also bears 

 several small leaves and behaves like 

 a fruiting branch of a navel orange 

 tree. 



Thorns in citrus trees, in cultivated 

 orchards, are nuisances. They inter- 

 fere with the work of picking the fruit, 

 in that they are likely to injure the 

 unprotected hands and arms of the 



pickers. Instances are known where 

 citrus pickers have sufifered serious in- 

 juries from thorns, resulting in the loss 

 of fingers, hands and eyes. Further- 

 more, the thorns are undesirable, as 

 they frequently puncture the nearby 

 fruits. These punctures offer oppor- 

 tunities for the entrance of various 

 fungus diseases which often result in 

 decay and loss of fruit. This decay 

 may take place on the tree, but more 

 frequently occurs after the fruits have 

 been packed for the market. For these 

 and other reasons citrus propagators 

 and breeders have endeavored to pro- 

 pagate thornless strains. 



It is a well-known fact that the size 

 and number of thorns vary on the dif- 

 ferent trees of a variety and often on 

 the different branches of the same tree. 

 Through the selection of buds for pro- 

 pagation from thornless limbs, or those 

 having small thorns, considerable prog- 

 ress has been made in isolating thorn- 

 less strains, or strains in which the 

 trees produce few and no thorns. 



The variation of thorns in the citrus 

 varieties, and in the strains of the same 

 variety, is convincing evidence as to 

 the heredity of this tree characteristic. 

 The variation in thorniness on the trees 

 of the same strain furnishes a basis for 

 the isolation of thornless strains 

 through systematic bud selection. 



315 



