RESIN PASSAGES 139 



detail, in so far as these structures are not passages but cysts ; 

 while he also appears to have overlooked the occurrence of simi- 

 lar structures in the three other species mentioned. In all of 

 these cases the cysts are contiguous and disposed in tangential 

 rows of considerable length, either in the summer wood (A. con- 

 color and A. nobilis), in the outer spring wood (A. firma), or in 

 both the spring and summer wood (A. bracteata). Such varia- 

 tions appear to be of no specific value, conforming as they do 

 to similar variations in the zonate distribution of the resin cells. 

 It is found, however, that in only one case (A. concolor) are 

 these cysts associated with isolated resin cells. In the three 

 other cases the resin cells are entirely wanting, a relation 

 which is strongly suggestive of their replacement by the cysts. 



Sequoia sempervirens is the only species of that genus which 

 develops resin cysts in the secondary wood, though Jeffrey (24) 

 has shown that such structures are normal to the primary wood 

 zone of S. gigantea, and not elsewhere. As already shown, such 

 cysts are much more highly organized than those of either Tsuga 

 or Abies, though they are similarly contiguous and even coa- 

 lescent, and form extensive tangential rows in the initial layer of 

 the spring wood of distant growth rings. They form a much more 

 prominent feature than in any of the preceding species because 

 of their generally larger size and the greater extent of the series 

 in which they lie. Unlike Abies, however, there appears to be no 

 diminution either in the number or the extent of the prominent 

 resin cells, which are often intimately associated with the cysts. 



The normal course of development for such cysts as are thus 

 described is subject to special alteration under conditions which 

 involve an unusual stimulus to growth, and under such circum- 

 stances they may become definitely associated with, or may 

 even be regarded as indicative of, pathological conditions. Thus 

 Anderson (1, 28-29) has shown that such cysts are definitely 

 developed in association with the formation of witches' brooms 

 in Abies firma. Under such circumstances the cysts become 

 much larger, more distant, and more numerous than in normal 

 growth, but they form well-defined tangential rows in the earlier 



