RESIN PASSAGES 137 



3. Resin cells when aggregated beyond a certain point develop schizoge- 



nous intercellular spaces, which form either central closed cysts or 

 central canals of indeterminate length. 



4. The structure of the cyst or passage always presents the same sequence 



of elements, and the work of the reservoir is then divided between 



a. the tracheids, which provide nutrition for the secretory cells ; 



b. the secretory cells or epithelium, in which the formation of the 



resin takes place ; 



c. the cyst or canal, which provides an outlet or storage reservoir for 



the surplus product ; 



d. the thyloses, which may impede the proper storage of the resin, or 



which may individually serve the purpose of storage. 



So long as the formation of resin is not excessive it is stored 

 in the cells where produced. This is true of all isolated resin 

 cells as well as of many which enter into the composition of 

 complex cysts and passages. When the resin is excessive, how- 

 ever, the surplus is excreted into specialized reservoirs of the 

 form of closed cysts or of canals, and we are led to interpret 

 the appearance of these structures in the higher Coniferae as a 

 response to such needs. The development of the resin passages 

 will thus be seen to stand in direct relation to the capacity of 

 the plant as a resin producer, --a fact which is otherwise appar- 

 ent from our knowledge of the general capacity of the different 

 genera as resin producers, and from a comparison of this feature 

 with their known position in the line of descent. 



