216 Plant Biology 



Internally the mature region of a root (Fig. 57) consists of (1) a cen- 

 tral stele (Gr. stele, pillar), (2) a surrounding cylinder of tissues called 

 the cortex, and (3) the external epidermis. These can be observed in a 

 cross section. 



The stele in the center is composed of (1) xylem, (2) phloem., (3) 

 parenchyma, and (4) pericycle. The xylem is in the form of a + and is 

 composed of thick-walled cells (tracheids and vessels) of various sizes; 

 the phloem is located in the angles between the strands of xylem and 

 consists of sieve tubes with their companion cells; the parenchyma tissues 

 lie between the xylem and phloem and are composed of rather large, 

 thin-walled cells; the pericycle (Gr. peri, round; kyklos, circle) sur- 

 rounds all of the above tissues and is in the form of a cylindrical sheath 

 (ringlike in cross section), being composed of one or several layers of 

 thin-walled cells. 



The cortex is composed of endodermis and parenchyma tissues. The 

 endodermis (en do -der' mis) (Gr. endo, within; derma, "skin") is the 

 innermost, single layer of cortex cells. The parenchyma tissues are just 

 external to the endodermis and are composed of rounded cells of various 

 sizes. 



The epidermis (ep i -der' mis) (Gr. epi, upon; derma, "skin") covers 

 the root and is one cell thick. Certain epidermal cells may possess pro- 

 jections known as root hairs to increase the absorption of the root. 

 Growth of the root occurs toward the tip rather than in the region with 

 root hairs, so that the latter are not injured as the root is pushed through 

 the soil. 



The composite flower of the sunflower (Fig. 57) is composed of nu- 

 merous individual flowers grouped together so as to form a head which 

 resembles a single flower in a general way. The small flowers are borne 

 on a disklike peduncle (pe -dung' el) (L. pedunculus, small foot) . 



At the edge of the flower-bearing disk are two or more spirals of o\er- 

 lapping, flat, green bracts (L. bractea, thin plate). On the face of the 

 disk is an outer circle of closely packed flowers, each in the axil of a 

 small bract (modified leaf). The two types of flowers on the head are 

 (1) ray flowers, forming one or two rows at the edge, and (2) disk 

 flowers, forming the remainder of the head. The marginal, ray flower 

 consists of a strap-shaped corolla, one side of which is modified into a 

 broad, flat structure. The stamens and style of the ray flower may be 

 abortive. These marginal ray flowers may be sterile or they may contain 

 only pistils. Each inner, disk flower consists of a wedge-shaped, hollow 

 receptacle partly enclosing the ovary which contains one functional ovule. 



