10 ANATOMY OF THE GYMNOSPERMS 



genus may be differentiated from the other two by the very 

 simple, constant, and well-defined characteristics found in the 

 presence of resin passages and fusiform rays. Yet once more, 

 Pseudotsuga, Larix, Picea, and Pinus fall into a natural group 

 characterized by the presence of resin passages, and within the 

 group differentiation of the individual genera follows on natural 

 and simple lines. This is particularly true of Pseudotsuga and 

 Pinus, in each of which the generic characters are so well defined 

 as to leave no room for doubt ; while yet once more, Pinus may 

 be subdivided into well-defined groups, or subgenera, representa- 

 tive of the soft and hard pines. Abies and Tsuga are differen- 

 tiated by the position of the resin cells and the character of the 

 terminal walls of the ray cells ; Sequoia and Taxodium are sep- 

 arated by the terminal walls of the ray cells, the character of 

 the bordered pits on the lateral walls of the ray cells, and by the 

 distribution of the resin cells ; Cupressus and Thuya are differ- 

 entiated by the terminal walls of the ray cells, the distribution 

 of the resin cells, the form of the pits on the lateral walls of the 

 ray cells, and the form of the ray cells in tangential section. 

 These principles are applicable to all other genera, and the key 

 as now presented affords a trustworthy guide. 



For the species the question has been found to involve much 

 greater difficulties, especially with reference to the genus Pinus, 

 in which the number of essential elements increases greatly, 

 while extreme variation also introduces a factor which adds 

 much to the complexity of the problem. Two sources of error 

 were early recognized as probable, --( i ) incorrectly drawn or 

 incomplete diagnoses, and (2) deviation from the selected type. 

 The problem was to elaborate an analytical key of such com- 

 pleteness as to eliminate any such errors, and then apply to 

 it a test which would prove the extent of its accuracy, em- 

 ploying the data so obtained in further corrections if necessary. 

 For the purposes of a critical test I was furnished with care- 

 fully selected material by Mr. Jack and Dr. Fernow to the ex- 

 tent of eighty-five specimens representative of fourteen genera 

 and forty-nine species. The specimens received from Mr. Jack 



