542 Rivett . — The Anatomy of Rhododendron ponticum, L., and of 
to variations in their cross-area. In Shoot L, which has very long internodes 
and appears rapidly grown, the range between the maxima is very slight — 
7*9, 8*i, and 8-5 being the figures found in successive yearly segments : the 
readings at the joints are 6*8 and 7*4 and the rich conducting-tissue is 
maintained until the last two internodes. In Shoot M the figures are 
normal in the lower yearly segments, but the one-year-old shoot was very 
Diagram XI. Internodes numbered from base to apex. (Scale, 5 internodes = 1 cm.) 
soft and there is not the usual displacement of vessels by fibres in the joint 
between the one-year-old and two-year-old segments. 
Curve D (average area of cavities in sq. /x). In both rhododendron 
and holly the curve descends from base to apex, indicating that the average 
cross-area of the vessels is largest at the base of the shoots. In the lower 
internodes, the new vessels which replace the basal fibres are small in size 
and their number helps to decrease the average area, though there are still 
numerous large vessels present. In the leafy region of Rhododendron 
ponticum a great number of larger vessels are found close outside • the 
primary xylem, and this causes an increase in the average size, which 
