246 THE AMERICAN NATURALIST. [Vol. XXXVIll. 



Miss C. M. Derick, I wish to extend my appreciation of the 

 frequent and vahiable assistance she has rendered. 



In the genesis of the vascular system, the elements first 

 differentiated from the generativ^e tissue, constitute the primitive 

 elements now generally recognized in accordance with the desig- 

 nation of Russow as protoxylem. These elements are tracheids, 

 and in the Coniferales as also in the Ginkgoales, they always 

 occupy a position immediately external to the pith and therefore 

 on the inner face of the zone representing the growth of the 

 first year, but they are not repeated in the formation of xylem 

 structure in subsequent years. The primitive character of such 

 tracheids is therefore indicated, not only by their position and 

 relation to development of other parts, but it is also exhibited by 

 their occurrence in plants where the vascular system is of a far 

 more simplified type and of which they constitute relatively more 

 prominent features. In the Equisetine^, both fossil and recent, 

 they are conspicuous elements of the vascular structure, being 

 found within the Hmits of the carinal canal. They similarly 

 occur in the Ophioglossaceae and elsewhere among the more 

 prmiitive of the vascular plants. The general evidence, then, 

 which may be derived from a comparative study of various types, 

 tends to enforce the idea that, originating as a primitive form of 

 the wood structure, and more or less common to all the vascular 

 plants, they are relatively predominant in the lower forms, with a 

 tendency to obliteration through replacement or modification in 

 the higher types, where their presence may be held to represent 

 a survival of ancestral characters. This view gains additional 

 support from a study of the peculiar structural variations which 

 t laractenze such tracheids, and the progressive modifications 

 which they have been found to undergo in relation to the 

 development of the secondary xylem. 



The protoxylem elements are distinguished by the presence of 

 turn, spiral bands disposed upon the inner surface of the primary 

 as to afford a substantial measure of 

 1 support. These ribbon-like bands represent second- 



cchanical s 



