THE WOODY DICOTYLEDONS 383 



of the aggregate ray, described above, into large homogeneous 

 masses of radial parenchyma as a consequence of the parenchyma- 

 tous modification of the separating fibers. This condition, known 

 as the compound ray, is found in relatively few dicotyledonous 

 woods, and these are ordinarily regarded as low in the systematic 

 scale. In woody types, moreover, it very readily passes into the 

 antecedent aggregate condition. A third and the commonest 

 condition of organization of the radial parenchyma in dicoty- 

 ledonous woods is presented by the diffuse condition. Here the 

 original aggregation, instead of retaining its identity or passing 

 into the compound state last described, spreads out in the manner 

 of a fan. This procedure results in the diffusion of the original 

 aggregations of rays evenly throughout the structure of the wood. 

 As a consequence of this phenomenon the organization of the 

 wood in the mass of dicotyledons is characterized by the presence 

 of abundance of rays which are of mediocre width. Sometimes 

 the rays of this type are nearly equal in size, but very generally 

 they range from extremely small to moderately large. Now and 

 then, however, as, for example, in the wood of beeches of the North- 

 ern Hemisphere and in the genus Platanus, extremely large rays 

 are found, readily distinguishable from those of the oak type 

 by the fact that they grade almost imperceptibly into radial 

 parenchymatous bands of mediocre dimensions. In the typical 

 compound ray such as occurs in certain species of Quercus, Cas- 

 uarina, and the Ericaceae the large ray is in sharpest contrast to 

 the primitive uniseriate condition. 



In the foregoing paragraphs a general account has been sup- 

 plied of the various anatomical features of dicotyledonous woods, 

 and an attempt has been made to indicate the primitive condition 

 in connection with each category of structures. It must not, 

 however, be supposed that a primitive condition of organization 

 in regard to any one of the categories described in the preceding 

 pages necessarily indicates for a given group a low position in the 

 evolutionary scale. Taken altogether, nevertheless, they supply 

 extremely valuable testimony from the standpoint of the doctrine 

 of descent and on the whole the best available in the present state 

 of our ignorance regarding fossil ancestors of the angiosperms. 



