60 DESCRIPTIVE CATALOGUE 



the medullary ray, which, in consequence, attains the highest 

 value in this respect [classification] and affords differential 

 characters of wide range, great prominence, and easy recog- 

 nition, and is of primary importance in. the differentiation of 

 groups, genera, and species; and, as a general summary, the 

 utility of these characters for such purposes is approximately 

 indicated in the following tabulation : 



"(I) Rays (tangential) of two kinds. 



(2) Eay-tracheids. 



(3) Pita on the lateral walls of the ray simple or 



bordered. 



(4) Terminal walls of the ray-ceils thin and entire or iQ ener j c 



locally thickened. 



(5) Form and character of the ray-cell (tangential). i 



(6) Form and size of the pits on the lateral walls of 



the ray-cells. 



(7) Ray-traclieids denticulate or reticulated. ) 



(8) Direction and form of orifice of pits on the lateral ! , -g , .. 



walls of ray-cells. 



(9) Upper and lower walls of ray-cells. 

 (10) Ray-tracheids interspersed or marginal. 

 <11) Disposition of pits (radial). 



(12) The number of pits per tracheid. ) 



The second half of Penhallow's book gives systematic series 

 of useful diagnoses of nearly all the living and fossil genera, 

 and of the American species, but he has no short general key 

 such as is supplied by Gothan. 



The value of the study of reversions to archaic character 

 under the stimulus of wounding has been much accentuated by 

 Prof. Jeffrey and his pupils recently, particularly in connection 

 with the formation of resin-canals and ray-tracheids (see Jeffrey, 

 1903, 1905, etc.). In addition to this, Jeffrey (1908) records 

 the presence of ray-tracheids due to a wound, in a living plant 

 of Cunninyliamia sinensis, where they would not normally occur, 

 and also has found ray-tracheids besides traumatic resin-canals 

 in a Miocene fossil attributed by him to Sequoia (1904), as well 

 as in the living species (1903). 



The elaboration by many students of the recent work on 

 woods, has accumulated data illustrating the immense variety 

 and also the intermingling of detailed characters in the various 

 i'amilie.s, so that very minute points ol difference have now to be 



