170 



GYMNOSPERMS 



throughout the year. Near Jalapa, Mexico, there are places where 

 some particular dicotyl may not show any growth-rings; while the 

 same species, 10 miles away, may have well-defmed annual rings. 

 In this case, the rings are absent where the forest is wet throughout 

 the year; while, a few miles away, outside the forest, there is a well- 

 marked alternation of wet and dry seasons, a condition which pro- 

 duces growth-rings. 



Coenoxylon (probably Permian) has distinct growth-rings. Pen- 

 HALLOW''''' divided the species of Dadoxylon (lower Carboniferous) 



.JBKi(JK« 



Fig. 180. — Cordaites sutclijjii: transverse section of stem; m, pith; .v, xylem; p, phlo- 

 em; c, cortex; /, leaf trace. — From a photograph by Land of a section made by Lomax. 

 From Coulter and Chamberlain, Morphology oj Gymtwsperms'^^ (University of Chi- 

 cago Press). 



into two groups, one characterized by the presence, and the other by 

 the absence, of growth-rings. A similar treatment of the plants of 

 the Jalapa region would be interesting, for individuals would prob- 

 ably change from one group to another, if they should be moved to 

 the corresponding locality. 



In the specimen shown in fig. 182 there are distinct growth-rings, 

 and so many of them that it seems necessary to interpret them as 

 regularly recurrent, perhaps seasonal. When only one or two rings 

 appear, where rings are not usually present, they may be due to in- 

 jury. If most of the top of the plant should be broken off, the growth 

 might nearly stop; and as a new top developed, growth would be 

 resumed, resulting in the formation of a ring, just as in Dioon cdidc a 

 ring is formed when a prolonged dormant period is followed by a re- 

 sumption of growth. 



