24 



FOSSIL FORESTS OF YELLOWSTONE PARK. 



tions — that is, they are photomicrographs — and have not been re- 

 touched in any manner. Figure 12 shows a transverse section of the 

 wood of the large redwood trunk that has been so often mentioned 

 (see title page). The section is cut through one of the growth rings, 

 which consists of 12 or 15 row^s of very thick-walled cells. The large, 

 regular thin-walled cells, which begin abruptly above the growth 

 ring, belong to the spring wood — that is, the wood first formed after 

 growth starts in spring, when the supply of nourishment is abundant. 

 If there is sufficient moisture and all conditions are favorable this 

 vigorous growth of wood cells may continue without interruption 

 until the approach of cold or dry weather, but not infrequently there 

 may be a brief shortage of moisture, and this is reflected in the for- 

 mation of a few row^s of thicker-Availed cells. Such a condition may 

 be observed in the present specimen, in which a slight, partial ring 

 may be seen at some distance above the main ring. 



The very perfect preservation of the wood of one of the pines 

 {Pityoxylon amethystinum) is shown in figure 13, a view of a sec- 

 tion cut through a part of a growth ring and into the spring and 

 summer wood, the rings in this species being so broad that it is 

 impossible to show a complete one. The opening near the bottom of 

 the figure shows one of the large resin ducts, which, in the living 

 Avood, is filled Avith the " pitch " that so readily exudes when a branch 

 is cut or broken. A longitudinal section of the other species of pine 

 {Pityoxylon aldersoni) is shown in figure 14. The many little rows 

 of superimposed cells in the midst of the long avoocI cells are the cut- 

 off ends of Avhat are knoAvn as medullary rays — that is, the little 

 plates of cells that connect pith and bark. One of the resin cells cut 

 in the long direction is shoAvn near the center of the figure ; the con- 

 tents are much darker than that of the Avood cells. 



The A^ery great difference betAveen the sections of coniferous wood 

 just described and the Avood of a deciduous tree is brought out in 

 figure 15, which is a longitudinal section of a laurel {Laurinoxylon 

 pidclwnm) . In this the Avood cells are relatiA^ely much smaller and 

 shorter, and the medullary rays are in several irregular roAVs. The 

 large dotted duct near the middle of the figure is a feature not 

 present in coniferous trees. 



SPECIES REPRESENTED. 



An enumeration of the kinds of trees that are represented by the 

 Avoods in the fossil forests of the YelloAvstone National Park will 

 naturally be demanded. A superficial or macroscopic examination of 

 these trunks would not permit a close decision as to the kind of wood ; 

 in fact, it would hardly be possible to do more than separate them by 

 this means into coniferous and dicotyledonous trees. But by study- 



