Anatomy and Histology. 



115 



character of the precipitation, as elsewhere described. The evidence gives 

 strong support to the view expressed by Holtermann (1907) that the ring- 

 structure of the wood is correlated with cessation and resumption of tran- 

 spiration. While it is not clear why an annular structure within the annual 

 ring is present in the wood of irrigated plants, it is quite possible that 

 it is due to stimulation by successive irrigations. These considerations 

 show that it is practically very difficult to determine the age of a plant 

 by counting the rings, and this is rendered still more so by their short 

 radial measurements. In the case before us (plate 33, fig. 2) 10 rings 

 are counted on a radius of 2 mm., so that the rings, taken altogether, 

 have an average thickness of 0.2 mm. Excluding the innermost (the first 

 season's growth) and the deepest, the rings vary from 0.06 to 0.3 mm. 

 approximately. This, coupled with their frequently great irregularity and 

 indistinctness (Ross, 1908), makes them difficult of recognition. 



The suggestion has been indicated inferentially in this connection by 

 Ross that the age of a stem is to be inferred from the number of secondary 

 canals and stratifications of the secondary cortex. A stem examined by 

 him, 19 mm. in diameter, showed eight zones of canals and of alternating 

 phloem layers, and he agrees with Endlich that the stem was about ten 

 years old. I feel quite sure, however, that this inference is far from jus- 

 tified. For example, the stem from which plate 33, fig. 2, was taken 

 was certainly over four years of age, and as certainly eight to ten. There 

 were only four rows of secondary canals. In a stem with a radius of i cm. 

 I count at least ten cortical zones, while there are but five in another cor- 

 tex of the same thickness. These, together with the further fact that 

 under irrigation a seedling in five months developed five rows of second- 

 ary canals, show that the number of canals depends upon the rate of 

 growth and not upon the number of seasons, and in field plants the num- 

 ber of rows of canals is, roughly, a third to a half less than the age of the 

 stem in years. A cortex 5 mm. thick, exclusive of the cork, taken from a 

 very old plant, about forty years of age, shows about twenty canal zones. 

 Some had of course been cut out by periderm, but scarcely as many as 

 twenty. 



