RESIN CANALS IN WHITE FIR 



355 



older twigs the large central cells of 

 resin canals surrounded by longer cells 

 of the same kind contain small globules 

 of resin, and rarely a uniform thick in- 

 ner layer. 



The general structure of resin canals 

 is exactly alike in all parts of the stem. 

 Transverse sections show that the cells 

 in the central portion of the canal have 

 wide cavities and are surrounded by 

 shorter resin-containing cells. The 

 outermost cells are longer and contain 

 starch in winter, and in the beginning 

 of summer contain volatile oil. Such 

 cells are always in direct communica- 

 tion with the pith rays. 



Resin canals originate from rows of 

 wide cells above referred to, or by the 

 gradual disorganization of the central 



with their cross-walls partly or wholly 

 absorbed. In older twigs all gradations 

 of disorganized side and cross-walls oc- 

 cur, which gradually dissolve, and ca- 

 nals with smooth side-walls develop. 

 The cell-walls within the resin canal are 

 sometimes found intact (Fig. 9). The 

 simplest form of resin canals consists of 

 one perpendicular row of resin cells. 



The wood-parenchyma fibers in the 

 uppermost part of one-year-old twigs 

 contain starch during the winter, the 

 same as the pith-ray cells. In older 

 wood the starch is replaced by a semi- 

 fluid resin in which there may be a few 



cells of a group. Seen in a longitudinal 

 section, the canal that is filled with 

 resin appears to have been formed by 

 the absorption of the walls of resin 

 cells. Such sections show that resin 

 canals are not continuous, but that 

 there are places where the cross-walls 

 have not been wholly absorbed. Twigs 

 from five to ten years old are best for 

 studying the development of resin 

 canals, because in older wood they are 

 already fully developed. If the canals 

 consist simply of wide central cells, such 

 elements may be seen either in their 

 original form or with their cross-walls 

 perforated or totally absorbed. In this 

 stage the side-walls have undergone 

 very little change, and the canal merely 

 consists of one or more rows of cells 



st.c. 



-.>. 



grains of starch. After growth has 

 started in early spring the starch grad- 

 ually disappears, and the cells become 

 filled with a fluid mass which is later 

 transformed into a yellowish or reddish- 

 colored resin. The resin gradually 

 hardens and forms an inner layer or oc- 

 casionally fills the entire cavity. Longi- 

 tudinal sections of young twigs show 

 perpendicular rows of wood-paren- 

 chyma fibers that contain starch 

 grains in the uppermost cells, while 

 the lowermost ones contain starch and 

 resin or resin only. The starch grains 



f O 



which originally filled the cell cavities 



