3 18 IRE POPULAR SCIENCE MONTHLY. 



ter of the stem. This is determined by the shortness of the diam- 

 eter of the lowermost segments. It goes without further saying 

 that the annual rings in exogenous (outside growers) stems vary 

 in age from the youngest upon the outside to the oldest at the 

 center. The point for us to determine is the lack of uniformity 

 in the wood and why that lack is somewhat regular. In other 

 words, the woody tissue of a stem is heterogeneous only within 

 certain limits. Thus in the wood shown in Fig. 1 there are thirty- 

 nine rings, and the tree for our purpose may be considered forty 

 years old in round numbers. Twenty of these rings, or the older 

 half, show a marked color, being much darker than the super- 

 imposed twenty years of annual deposits. Several other things 

 are shown by this section, and we can well dwell upon this speci- 

 men, as it illustrates facts that are common to nearly all trans- 



FIG. 5. CROSS SECTION OF PIN OAK. Negative. 



verse cuts of wood. The rings, for example, are not all of the same 

 width, those formed while the tree was passing from its fifth to 

 the twentieth season being the largest, but even among these 

 there is a wide variation. Thus, ring fifteen from the center is a 

 narrow one, followed by one of unusual width. For the last ten 

 years the rings have been more uniform and much thinner than 

 twenty years earlier. There may be one or more of many reasons 

 for a ring being unusually thin, as, for example, a short season, 

 one lacking in moisture or having an excess of it, injury from 

 frost, fires, insects, or parasitic fungi. The decrease in thickness 

 toward the outside of the papaw may be due to insufficient nutri- 

 tion, approaching old age, etc., but in this connection it must not 

 be overlooked that the amount of actual wood deposited may be 

 more in a thin ring at the fortieth year than in a comparatively 

 thick one at the tenth year, the surface covered being so much 



