BRIEFER ARTICLES 



DEPRESSED SEGMENTS OF OAK STEMS 



(with four figures) 



In a recent paper, Miss Langdon' questions certain statements of the 

 writer' in regard to the deeply depressed or sunken segments which 

 occur commonly in stems of Quercus. She states (p. 321): 



From observations of transverse sections of twigs from Quercus alba, 

 Q. bicolor, and Q. macrocarpa I find that there is evidence of retardation in 

 growth of the tissues in the immediate vicinity of the wide rays, especially 

 noticeable in the marked dipping in of the annual rings where they cross the 

 large rays. However, aside from a few extreme cases, this checking influence 

 of the wide foliar rays does not explam the 5 conspicuous depressions so char- 

 acteristic of the wood of Quercus. 



In discussing the topographical features of the stem of the oak, it is 

 essential to distinguish between two different factors which have modi- 

 fying effects upon the outline of the secondary xylem. I refer to the 

 arrangement of the primary elements and the development of multi- 

 seriate rays. The effects of these two factors, or complexes of factors, 

 may be studied most satisfactorily in plants where they occur independ- 

 ently. For example, in Castanea dentata and Populus halsamifera, 

 which normally have only uniseriate rays, the primary elements have 

 the stellate arrangement that is characteristic of Quercus. In the inter- 

 nodes of normal stems of these plants, the first formed secondary ele- 

 ments form a layer of undulating or stellate outline, but at the end of 

 two or three growing seasons, frequently earlier, the outer periphery of 

 the secondary xylem tends to be circular (fig. i). In other w^ords, the 

 early lobed or stellate form of the cambium soon becomes evanescent, 

 and its effects upon the shape of the stele are quite transient. 



The modifying influences of the second set of factors, acting inde- 

 pendently of the first, may be seen quite clearly in the stems of certain 



'Langdon, LaDema M., The ray system of Quercus alba. Bot. Gaz. 65: Z'^Z- 

 323. 1918. 



2 Bailey, I. W., The relation of the leaf trace to the formation of compound raj-s 

 in the lower dicotyledons. Ann. Botany 25:225-241. 1911. 



438 



