THE GROWTH KING ? >l 



and not infrequently is almost obliterated. The summer ^vood 

 thus characterized is also distinguished by its greater hardness, 

 which in the Douglas fir imparts an almost flinty character to 

 the structure ; and commonly also, as exhibited in Pseudotsuga 

 Douglasii, Pinus palustris, P. resinosa, etc., by a considerable 

 amount of resinous matter, which, by reason of its definite color, 

 establishes a more or less striking contrast between the two 

 regions of growth. It is therefore obvious that the demarcation 

 of the growth rings depends upon the direct apposition of spring 

 and summer woods, each of which is characterized by special 

 structural and other physical features. 



The transition from the spring to the summer wood is gradual 

 when there is no sharply defined limitation of the two zones, but 

 the one seems to merge into the other by a series of more or 

 less insensible gradations. This is exemplified in its typical form 

 in the Cordaitales, and among the Coniferales it is a feature 

 of the great majority of species. Under such circumstances the 

 internal limits of the summer wood can be determined only 

 approximately, and they are necessarily established where the 

 greatest alteration of structure and color occur. Less frequently 

 the transition is abrupt, as in the hard pines, notably P. palustris, 

 or in the Douglas fir (plate i). There is then a strong contrast 

 between the two structural regions. A further variation of this 

 relation is to be seen in those cases in which the somewhat grad- 

 ual transition from the spring to the summer wood is followed 

 the next year by a corresponding change to the spring wood. 

 This is of rare occurrence and is to be met with in only one case, 

 -Pinus Torreyana, in which also such double graduation seems 

 to arise only in the case of thick summer wood, since, when the 

 latter is thin, the transition is abrupt. Rarely the summer wood 

 exhibits a zonal development whereby it becomes doubled through 

 the interposition of a zone of thin-walled and rather large-celled 

 tissue. This finds its typical development in Taxodium distichum, 

 of which it is a characteristic feature. It may also be met with 

 occasionally among the higher Coniferales, especially among the 

 hard pines. It is a constant feature of taxonomic value. 



