Angiospermous Plants — Flowering Plants 209 



INDIAN CORN 



Zea mays (ze' a) (Gr. zea, corn). — Indian corn has an erect stem 

 from which adventitious roots are formed at the nodes (L. nodus, knob 

 or joint) to assist the true roots in the absorption of water and dissolved 

 materials from the soil as well as to assist in anchoring the plant (Figs. 

 58 to 60). Consequently, such unusual adventitious roots are called 

 "brace roots" or "prop roots." Roots which develop directly from stems 

 or leaves are called adventitious. 



Corn has a typical monocotyledonous stem with numerous vascular 

 bundles (Fig. 60) scattered throughout the stem which is composed of 

 parenchyma cells (par -eng' kima) (Gr. para, beside; engchyma, infu- 

 sion) of various sizes and shapes. These can be observed in a cross sec- 

 tion. The external cover of the stem consists of a layer of epidermis 

 whose cells are relatively small and thick walled. Beneath the epidermis 

 is a narrow layer of (mechanical) sclerenchyma tissue (skier -eng' kima) 

 (Gr. skier OS, hard; engchyma, infusion) whose cells are small and thick 

 walled with lignin (lig' nin) (L. lignum, wood). Each vascular bundle 

 is surrounded by a sheath or layer of thick-walled (mechanical) scler- 

 enchyma tissues. Internally, each bundle consists of (1) phloem (toward 

 the periphery of the stem) and (2) xylem (toward the center of the 

 stem). There is no meristematic cambium separating the phloem and 

 xylem, as in dicotyledonous stems, so there can be no indefinite increase 

 in size after the primary tissues are mature. Bundles lacking cambium 

 are called "closed" bundles because of their inability to grow indefinitely 

 (Fig. 60). 



The phloem of a mature bundle conducts liquids downward and con- 

 sists of regularly arranged, nonnucleated, sieve tubes and companion 

 cells (Fig. 18). The sieve tubes have their adjacent end walls supplied 

 with a perforated sieve plate. Often a^ narrow, thin-walled, elongated, 

 nucleated, companioin cell lies parallel to the sieve tube. 



The xylem conducts liquids upward and consists of two large vessels, 

 with pitted walls, located next to the phloem. Between these two vessels 

 are a few, hollow, one-celled tracheids (Fig. 18). The innermost part 

 of the xylem contains one or two vessels whose walls have ring-shaped 

 or spiral thickenings. Between the latter vessels and the sheath of me- 

 chanical tissue is a large, hollow intercellular space. 



The leaves of corn are characterized by numerous, main veins running 

 parallel to the long axis, and all connected by a network of fine, incon- 

 spicuous, branches (Fig. 60). The veins are actually vascular bundles 



