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(Fig. 4, b). The same process occurs in the transverse walls. Finally, 
the tangential walls become lignified, the inner undergoing this 
change first (Fig. 4, c and d). In older leaves irregular deposits of 
suberin can be found on the tangential walls, the typical structure of 
the adult form of the fascicular leaf thus being attained. 
Fig. 4. Pinus sylvestris L. ( a ) Transverse section of young leaf near the base. 
Endodermis regular but unthickened. (&) Transverse section of fairly 
young leaf. Radial walls only thickened. They consist of a lignocellulose 
core with a surface layer of suberin. ( c) Transverse section of older leaf. 
Radial walls lignified and suberised; inner tangential walls lignified. 
(d) Radial longitudinal section of older leaf. Transverse walls lignified and 
suberised; inner tangential walls lignified. en., endodermis; pal., palisade 
tissue; per., pericycle; s., suberin; lig. lignocellulose. ( x 240.) 
(e) The appearance of the endodermis as seen in passing from the 
dwarf shoot to the leaves. 
The evolution of the endodermis at the base of the leaves is 
shown by the series of diagrams in Text-fig. 5. No trace of an endo- 
dermal sheath is visible in the dwarf shoot. Towards the apex of this 
shoot a concentration of the cortical tissues occurs round the con¬ 
ducting cylinder, the cells becoming smaller and more densely packed. 
