534 Rivet t. — -The Anatomy of Rhododendron ponticum ) L., and of 
proportion existing between the area of the cavities and the area of the 
known sector of wood was obtained. By dividing the total area of cavities 
by the square of the linear magnification and combining this with the. true 
area of wood, the total area of the cavities in a complete transverse section 
was easily calculated. Further data which were calculated were the average 
area of the cavities and the number of cavities per square millimetre. 
Accuracy. Owing to the comparative smallness of the field under 
a high power of the microscope, only a small sector of the wood can be 
examined at once. Neither in Rhododendron ponticum nor in holly is any 
such sector typical of a whole transverse section. The variability of the 
wood on different radii is due in Rhododendron ponticum to (i) the two- 
sidedness of the stem, (ii) the existence of tracts of living cells associated 
with the insertion of the petioles ; in holly the variability is due to the 
living cells and small vascular elements associated with the insertion of the 
petioles. Theoretically, perfect accuracy can only be obtained by ascertain- 
ing the cross-area of every water-conducting element in every section ; the 
laboriousness of this being evident, it was found that the ^approximation 
arrived at by combining three or five sectors gave satisfactory results. 
Results . The results given by the above-described drawings and 
calculations are represented by a series of graphs. In all cases the inter- 
nodes numbered from base to apex are plotted along the horizontal line, an 
equal interval (o*2 cm.) being taken for each internode, irrespective of its 
length. A special vertical scale is chosen for each set of figures according 
to the following plan : 
A. Total wood area in sq. mm. 
B. Total cavity area in sq. mm. . 
C. Percentage of wood occupied by cavities . 
D. Average area of cavities in sq. \x 
E. Number of cavities per sq. mm. 
i cm. represents 5 sq. mm. 
1 cm. represents 1 sq. mm. 
1 cm. represents 2 per cent. 
1 cm. represents 40 sq. y. 
1 cm. represents 200. 
By plotting points for each set of figures against the chosen internodes, 
a series of curves are obtained which show how they vary at different levels 
of the stem. The members of the series are indicated by the letters assigned 
to the plan of vertical scales put out above. The curves for the different 
shoots of Rhododendron ponticum are illustrated in Diagrams IV to XIII : 
the curves for the shoots of holly in Diagrams XIV to XVII. 
Curve A (total area of wood in sq. mm.). In general this is a smooth 
curve descending from base to apex. The descent is due to the general 
tapering of the stem and the increase in the relative area of the pith in the 
higher levels. In Rhododendron ponticum the descent is generally more 
rapid towards the base than in the middle and apical regions, but this is 
not generally apparent in the holly. Of the rhododendron shoots B 1 is 
the most massive, showing a wood-area of 32-2 sq. mm. at the base ; in 
general the leading shoots are thicker than the lateral : in some cases 
