532 THE AMERICAN NATURALIST. XXXVIII. 



guish such elements as parcncJiyjua tmcJicids in order to estab- 

 lish their proper identity and differentiate them from the wood 

 tracheids, which have a wholly different origin, as well as from 

 the ray tracheids, which have a wholly different location. It is 

 probable that the parenchyma tracheids also serve a similar pur- 

 pose to the ray tracheids with respect to the distribution of 

 nutrient fluids. The origin of the parenchyma tracheids as sug- 

 gested finds support in the statement of Eichler ( 1 1 ) that the 

 wood parenchyma arises through the activity of the cambium 

 cells, abundantly in the Cupressineae and Abietineae, forming in 

 exceptional cases the epithelium of the resin canals, since it at 

 the same time shows how the parenchyma tracheids arise and 

 how they may be intimately connected with the wood-paren- 

 chyma ; but it finds additional support in a knowledge of the 

 genesis and structure of the resin passage. 



In Sequoia and Abies we have two genera which are remark- 

 able for their transitional forms of structure, affording a fairly 

 clear conception of the genesis of the resin passage. In each 

 case there is a well defined tendency toward the aggregation of 

 the resin cells into compact groups which take the form of 

 longitudinal strands, enclosed on all sides by the accompanying 

 parenchyma tracheids. Under such circumstances the indi- 

 x idiuil I ells undergo a continual reduction in length until they 

 eventually become but two or three times longer than broad, 

 or they may even become isodiametric. This change is not 

 accompanied by any alteration in the thickness of the walls in 

 the earlier stages of development, but as a result of such a 

 shortening the effect is to bring about the concentration of a 

 greater number of simple pits within a given area. Such cells, 

 therefore, are always more strongly pitted than those which are 

 isolated and of greater length. When aggregates of this sort 

 have attained to a certain degree of development a line of 

 cleavage arises in the center of the mass and results in the for- 

 mation of an intercellular space which, according to Eichler (n), 

 always arises schizogenously. This space is short and either 

 isodiametric or but little longer than broad, the length coincid- 

 ing with the principal axis of growth. Such cyst-like reservoirs 

 or sacs represent the primitive form of the resin canal, and they 



