672 



A TEXTBOOK OF THEORETICAL BOTANY 



leaf of the Angiosperm (Fig. 671). The whole anatomy suggests strict con- 

 servation of the water supplies, and it is therefore spoken of as xeromorphic . 



The leaves are semicircular in cross-section, each pair having their flat 

 surfaces facing each other. Bounding the outside of the leaf is a very thick- 

 walled epidermis. This is covered on the outside by a thick cuticle. 

 Stomata are developed in longitudinal rows, on all sides of the leaf, and the 

 guard cells are sunken in grooves, well below the general surface of the 

 epidermis. 



Beneath the epidermis is a hypodermis, composed of one or two layers 

 of sclerenchymatous cells, interrupted by air spaces below each stoma. The 

 parenchymatous mesophyll is not divisible into palisade and spongy tissue, 

 but is made up of thin-walled cells, containing numerous chloroplasts and 



Resin canal 



Mesophyll 



Endodermis 



Transfusion 

 tissue 







^' ^n wl 



'^h^--^.---^. 



( '• : ■ 



i(^^*^ -^ilfi^ ■'' ■■■' W/ 





Ston\a 





m^>-- 



Fig. 671. — Pimis Sylvestris. Transverse section of a needle leaf. 

 The flat side is adaxial. 



rich in starch, whose walls show numerous infoldings, which project into the 

 cavities of the cells. 



These curious folds run in a circular manner round the cell and represent 

 rigid, non-growing, ring-like zones in the wall. When they are formed the 

 further growth of the cell causes the softer wall portions between the rings 

 to bulge out in balloon-like expansions, whose sides, in contact with each 

 other, give the appearance of folds. There is thus no actual inward growth 

 of the cell wall, which would have to be against the turgor pressure in the 

 cell and would be diflicult to understand. Around the outer part of the leaf 

 is a ring of resin canals similar in structure to those of the stem. 



In the centre of the leaf is a well-marked endodermis surrounding a 

 many-layered pericycle, within which are two vascular bundles, which run 

 closely parallel and unbranched from base to apex of the leaf. These two 

 bundles arise from a single leaf trace, and in some Pines the trace continues 



