218 Trans. Acad. Sci. of St. Louis. 



tlios: " In less favorable situations and in poorer soil it is 

 low, stunted, wide branched, and often covered with thorns." 

 Whether or not it will be found that the production of thorns 

 on the trunk is more apt to occur in individuals developing 

 under unfavorable conditions, I cannot say. 



It might seem that the production of these thorns is 

 governed to a certain extent by inherent individual tendencies. 

 I have not been able to account for it entirely on the basis of 

 environment, since individuals with or without them may be 

 found in the most widely differing localities, rich, moist 

 bottom land as well as the poorer, dryer soil of the hills. 



To find an explanation for one of the anomalies observed it 

 will be necessary to notice the ontogeny of the normal thorn. 

 Various explanations of its position and origin have been 

 offered. The following * seems the most plausible. In the 

 growing tip of the Gleditschia twig there is early differentiated 

 in the axis of the leaf primordium, the meristem of the axil- 

 lary shoot. As growth progresses this axillary bud is carried 

 forward by the elongation of the main axis. The region 

 below this axillary bud is surrounded in its earlier stages of 

 development by the somewhat clasping base of the leaf petiole 

 and here are formed a series of buds, beginning next the orig- 

 inal axillary bud and passing down the stem, the lowest bud 

 being the last formed. It will thus be seen that, although in 

 its mature form it is removed a considerable distance from the 

 axis of the leaf, the thorn has its origin in the original axil- 

 lary bud. In subsequent vegetative periods these buds de- 

 velop into foliage branches, thorns and flowers. One of the 

 buds usually develops into a short branch, about 3-6 mm. 

 long, bearing a rosette of once pinnate leaves which represent 

 a very important part of the photosynthetic surface of the 

 plant. In one of the two following years this branch usually 

 dies but it may be continued as one of the regular foliage 

 branches. 



While the thorns on the trunk are usually much branched, 

 all the buds produced on them usually do not develop. Fig. 35 



* See Delbrouck, Courad. Die Pflanzeu-Stacheln. Botanische Ab- 

 handlungeu aus dern Gebiet der Morphologie uud Physiologic. 2 : 4. — Kussel, 

 W. Eecberches sur les bourgeons multiples. Ann. Sci. Nat., Botauique. 

 vii. 15:95-202. — Shull, Geo. H. Accessory buds. Bot. Gaz. 21 ; 166-169. 



