40 ANATOMY OF THE GYMNOSPERMS 



one or the other intermediate form." It is probably a justifiable 

 inference from the preceding facts that the relation which exists 

 between the spiral tracheids of the protoxylem and the pitted 

 tracheids of the secondary xylem in the Coniferae is, in general 

 terms and from the standpoint of development, the same as 

 that exhibited between the lower and higher types of vascular 

 plants. 



Inasmuch as specimens derived from fossil woods or from 

 recent woods which have been employed for constructive pur- 

 poses will almost invariably represent some portion of the sec- 

 ondary wood only, it follows that in all but exceptional cases 

 the spiral structures so far considered will be entirely absent, 

 and in those few instances in which they may occur the de- 

 termination that they belong to the protoxylem region can be 

 made without difficulty. It is nevertheless true that in a few 

 genera definite spirals are to be met with in the secondary wood 

 structure of which they may then be characteristic features 

 throughout the entire extent of the growth rings, or they may 

 be more or less localized. Such spirals, which are obviously of 

 an exceptional nature, are features in the development of the 

 tertiary wall of the tracheid, and they are therefore character- 

 istics of thick-walled elements. In all their essential character- 

 istics of form and distribution they conform to the laws which 

 govern the spirals of the secondary wall, but they show a marked 

 tendency to obliteration through degeneration in the relatively 

 thicker walls. Thus in the genus Taxus or Torreya such spirals 

 are common to all the tracheids of the growth ring, but in Larix, 

 as also in Pinus tseda, in both of which the walls are relatively 

 thicker, the spirals are reduced to a vestigial form, being spo- 

 radic and in the one case distant, while in the other case the 

 individual spirals are only partially developed. This law is more 

 exactly and specifically illustrated in Pseudotsuga, where there 

 is a strong contrast between the thin-walled spring wood and 

 the very thick-walled summer wood. In the former the spirals 

 are perfectly formed and constant, and they bear a very strong 

 resemblance to what may be observed in the Taxaceae. In the 



