Nos. 451-452.] AjVATOA/V OF THE COMFERALES. 543 



such substitution does actually 

 noted that the second and third 

 in part, of tracheids. 



erallv elong 

 th the 



allel 



growth and completely 

 closed at both ends (Fig. 

 44). The epithelium 

 which immediately de- 

 fines the limits of the 

 sac generally consists of 

 short, cylindrical cells, 

 while in the second or 

 outer layer the cells be- 

 come much elongated 

 and several times longer 

 than broad. Beyond 

 this, the third layer con- 

 sists of parenchyma tra- 

 cheids readily distin- 

 guishable whenever the 



terminal walls lie near _ ^^.^ sempervirens. Radial section of a r 



the plane of section, or cyst, sliowing the epithelium (ep.) ; the central cyst 



otherwise as already in- Jl^lfhe^id^f the'sptU w^dT^?/^^^ 

 dicated. Certain devia- 

 tions from this typical structure require examination. The re? 

 sacs are placed in vertical series of indeterminate extent, but 

 varying intervals of such a nature that they max sometimes 

 separated only by a rather thick wall of short resin cells, 

 other times they are somewhat distant and separated by 

 extensive vertical tract of resin cells. From this it is obvic 

 that in any given plane of section there will be a great divers 

 of aspects presented, but in the main exhibiting structural gra^ 

 tions in the development of the reservoir i 

 In some cases thick-walled cells of circular 



