598 



WEED FLORA OF IOWA 



Fig. 453. I. A cross section of a leaf of Feppergrass (.Jjepidium) showing 

 the upper epidermis (e), the lower epidermis (S), stoma (s), the cblor- 

 enchyma (c) consisting of closely placed palisade cells (p) and more 

 loosely placed spongy tissue (f), and a vascular or conductive tract (v) 

 with bundle sheath (b), hadrome or xylem (h), and leptome or phloem (1). 



II. Surface view of stoma from Easter Lily ; g, the kidney-shaped guard cells en- 



closing the stomatal aperture (s) ; b, the subsidiary cells. 



III. Cross section of stoma ; g, guard cell ; s, central slit ; o, outer slit ; 1, 



inner vestibule ; c, stomatal cavity ; b, subsidiary cell. 



IV. Surface view of a grass stoma (Poa pratensia) showing the guard cells 

 (g). with their dumb-bell-shaped Imnlna ; b, subsidiary cells with prominent 

 nucellus (n). 



v. Median cross section and cross section through end of stoma of Poa annua; 



g, guard cell lumlna ; b, lutnina of subsidiary cells. 



VI. A cross section of a leaf of blue violet {Tiola oucullatay showing a single 



row of elongated palisade cells (p), and the loose spongy tissue (f). 



(Drawings after Cowles modified by Charlotte M. King.) 



A cross section of a leaf will show the interior filled with a mass 

 of thin walled cells containing green bodies {chloroplasts) . This 

 inner mass of thin walled cells is called the mesophyll and is the 

 food making tissue of the leaf. In the leaves of most weeds the 

 cells just under the upper epidermis are much elongated and stand 

 at right angles to the epidermis. These elongated cells are known 

 as the palisade-cells. Between the palisade-cells and lower epi- 

 dermis is the spongy tissue made up of irregularly shaped cells, 

 so loosely joined as to form a system of intercellular spaces which 

 permit the circulation of gases through the interior of the leaf. 

 In the lower epidermis are seen the stomata with the air cham- 

 bers beneath. Scattered through the mesophyll are the cross sec- 

 tions of veins and veinlets which form the frame work of the leaf 

 and conduct materials to and from the green working cells. 



