Angiospermous Plants — Flowering Plants 215 



secondary xylem may contain rigid, thick-walled, elongated cells with 

 pointed ends and are called wood fibers. 



A mature sunflower leaf (Fig. 57) consists of a broad blade attached 

 to the stem by a slender petiole. Net veins conduct materials through- 

 out the leaf. Photosynthesis is carried on by the chlorophyll which ab- 

 sorbs light energy and combines carbon dioxide and water to manufac- 

 ture organic compounds, with oxygen as a by-product. Some of the 

 stored energy is used by the plant for its various metabolic activities, while 

 some of the energy absorbed is used in the manufacture of the organic 

 foods. Capture and storage of light energy by chlorophyll-bearing plants 

 is a unique phenomenon. The leaf blade (Fig. 57) includes (1) an 

 external epidermis w^hose cells are often irregular when viewed from the 

 surface but rectangular in cross section; the outer walls of the epidermal 

 cells contain a waxy cutin; (2) a region of column-shaped palisade cells 

 beneath the upper epidermis which are compactly arranged and contain 

 many chloroplasts; (3) a layer of spongy tissue beneath the palisade layer 

 composed of irregularly shaped, loosely packed cells with numerous in- 

 tercellular spaces between them {chloroplasts are also present in the cells 

 of the spongy tissue) ; (4) a lower epidermis similar to the upper epi- 

 dermis except that it contains openings known as stomata for the ex-, 

 change of gases. Each stoma is bordered by two bean-shaped guard 

 cells which contain chloroplasts and regulate the size of the stoma. The 

 stomata open into intercellular cavities (substomal cavities) in which 

 gases may be exchanged with the leaf cells. Both epidermal tissues may 

 bear hairs; (5) veins which are vascular bundles composed of xylem and 

 phloem as in the petiole and stem; a sheath of variable thickness sur- 

 rounds the vein. 



An older sunflower plant has a primary root with lateral branch roots 

 which may have an extensive branching root system for anchorage and 

 absorption. 



The regions of a mature root, beginning at the tip, include ( 1 ) the 

 cup-shaped root cap which protects the root as it is pushed through the 

 soil; (2) the meristematic region (growing region) which is covered by 

 the root cap and is composed of small, similar, closely packed, rapidly 

 dividing cells; (3) the elongation region, just back of the growing region, 

 which is composed of cells which are increasing in length; (4) next the 

 maturation region in which the cells are diff"erentiated and are taking on 

 mature characteristics; and (5) following this the mature region in which 

 most cells have completed their development. All these regions can be 

 observed in a longitudinal section. 



