272 
Isabel Soar 
(a) The cotyledons. 
Six or seven of these are present and each is triangular in section. 
Stomata are found on the two radial faces only. There is no double 
hypodermal layer. The endodermis (Fig. 1) is present as an uninter¬ 
rupted sheath of rather large and fairly regular cells separating the 
conducting tissue from the mesophyll. Scarcely any infolding of the 
walls of the latter occurs and resin canals are absent. All the walls 
of the endodermis consist entirely of cellulose. Developing tracheidal 
cells are visible in the pericycle. 
Fig. 1. Pinus sylvestris'L. Transverse section of cotyledon showing unthickened 
endodermis. en., endodermis; pal., paiisade tissue; per., pericycle; 
tr., developing tracheidal tissue. ( x 240.) 
( b ) The primary leaves. 
These are distinguished from the fascicular leaves produced later 
by the fact that they grow singly on the stem. They are elliptical in 
section and bear stomata on both sides. The mesophyll is plicate and 
two resin canals are present, but there is no double hypodermal layer. 
The bundle is separated into two symmetrical portions by a large 
medullary ray. The endodermis (Fig. 2, a) appears as an unbroken 
sheath. The radial and transverse walls are thinly suberised. After 
prolonged treatment with concentrated sulphuric acid a slight amount 
of carbonisation is seen in the centre of these walls, and also in parts 
of the outer tangential walls, showing that a small amount of ligno- 
