352 



AMERICAN FORESTRY 



ready been stated, the contents of young 

 cells is gradually converted into an oily 

 mass and eventually into resin. Trans- 

 verse and longitudinal sections of older 

 shoots show that during the winter the 

 resin cells are filled with semi-fluid resin 

 occurring either in the form of a thin 

 layer or in small globules. 



For convenience of study, Dippel 

 grouped the resin-containing elements 

 in the wood of white fir into single resin 

 cells, large groups of resin cells, and 

 true resin canals. He briefly described 

 the structure of the dififerent groups, 

 their relation to the surrounding tissue, 

 as well as the form and function of the 

 individual cells composing the different 

 groups. The following are the chief 

 facts brought out by Dippel : 



DESCRIPTION OF GROUPS 



I. Among the single resin cells mav 

 also be included small groups of from 

 two to four resin cells (transverse sec- 

 tion) found in the wood of the roots 

 and stems (Fig. i). In the latter they 



:Ft^ 



usually occur among the thin-walled 

 tracheids of the early wood (spring- 

 wood of Schacht), and are seldom pres- 

 ent among the thick-walled cells of the 

 late-wood (fall- wood of Schacht). In 

 wide concentric zones of roots they oc- 

 cupy a similar position, but in narrow 

 zones they are found either among the 

 thick-walled tracheids of the late-wood 

 or in the transition zone between early 

 and late-wood. They are found in all 

 genera of the conifer?e except in Pinus 

 and Picea, and in structure are similar 

 to those of the wood-parenchyma fibers 

 in Dicotyledonous woods. The individ- 

 ual cells composing these fibers form a 



perpendicular row of elements that have 

 either horizontal or oblique cross-walls 

 and contain numerous simple pits. 

 Where the side-walls of such elements 

 are adjacent to other elements of the 

 same order or border on pith ray cells 

 the pits are invariably simple ; where 

 they are adjacent to tracheids the pits 

 within the walls of tracheids are bor- 

 dered, and within the walls of the wood- 

 parenchyma fibers are simple, semi- 

 bordered pits. The character of pits in 

 resin cells correspond exactly with 

 that of pith ray cells. The pit cavi- 

 ties within the walls of tracheids ad- 

 jacent to resin cells are invariably 

 smaller in diameter than those in the 

 contiguous walls of tracheids. The 

 lumina or central cavities of the resin 

 cells are somewhat smaller than those 

 of the surrounding tracheids which are 

 the elements forming the ground mass 

 of coniferous woods. The average 

 length of the resin cells is between 

 .30 and .35 mm., but they have been 

 found to vary from .15 to 1.05 mm. 



In older parts of stems the majority 

 of resin cells are completely filled with 

 resin, while others have thick layers 

 of resin deposited on their inner walls. 

 There are individual cells that are filled 

 during the winter with starchy matter 

 the same as the pith ray cells. In the 

 wood of ultimate twigs resin cells are 

 filled almost completely with starch. 

 With the beginning of the season's 

 growth the starch disappears and a 

 strong light-refracting solution that is 

 volatile and coluble in alcohol takes its 

 place (Fig. i). In the older portions 

 of the twig the number of starch- 

 containing cells diminishes and those 

 of oil and resin-containing elements in- 

 crease. In longitudinal section of one- 

 year-old twigs it is found that the cells 

 near the end of the twig contain starch, 

 while those farther back contain merely 

 an inner layer of resin. Here and there 

 starch grains and resin (Fig. 2, r. c.) 

 are often present in one and the same 

 cell. 



2. Large groups of rcsiti cells, with 

 which may be classed all those groups 

 that consist of from six-twenty or 

 more, are usually found just inside the 



