78 ORGANIZATION OF HIGHER PLANTS 



Exercise XV 



tissues, the xylem which transports water and 

 dissolved minerals upward from the roots, and 

 phloem which transports food materials from the 

 leaves to all parts of the plant. Phloem cells 

 usually do not have as thick walls as xylem, and 

 may be distinguished from them on this basis. 



Stem anatomy 



Begin by looking at the slides of the herbaceous 

 (green) angiosperm stems, alfalfa (Medicago), a 

 dicot, and corn {Zea mays), a monocot. Both 

 contain easily recognized vascular bundles of 

 both xylem and phloem. In dicots, however, the 

 vascular bundles are arranged in a ring, while in 

 monocots the vascular bundles are scattered 

 throughout the stem. This is one of the major 

 differences between mono- and dicots. 



A second significant difference is that dicots 

 may retain some meristematic tissue in the stem 

 (called cambium) which lies between the xylem 

 and phloem, and which may lay down new xylem 

 and phloem. Most monocot stems, on the other 

 hand, lack a cambium. 



Identify the following cell types in the stems: 

 epidermis, parenchyma, phloem, xylem, fiber 

 tissue, and cambium. 



Next study the cross sections of the 2- to 4- 

 year-old woody stem of the tulip tree (Lirio- 

 dendron). Here the cambium has produced new 

 (secondary) xylem and phloem. The earlier 

 (primary) xylem has been left behind as orderly 

 rows of cells (wood), but the primary phloem 

 has been crushed to a thin layer which lies just 

 beneath the epidermis. The xylem cells that 

 form in the spring of the year are bigger than 

 those that form in the summer and fall. The 

 latter also have thicker walls. These differences 

 account for the annual rings visible in a tree 

 trunk. 



You will find also some tangential and radial 

 sections of stems of Liriodendron and Thuja 

 (arborvitae). 



There will also be pieces of various woods 

 available, cut in different planes. Examine these, 

 correlating their grains with the microscopic 

 sections. 



Leaf anatomy 



Examine the fixed and stained cross section of 

 a leaf of privet (Ligustrum). Note the following 

 layers: 



1. Cuticle and epidermis of the upper surface 

 of the leaf. 



2. Mesophyll. This is made up of two layers, 

 the palisade parenchyma and the spongy 

 parenchyma. (Of what advantage are the inter- 

 cellular spaces in the spongy parenchyma?) 

 The veins (vascular bundles) are distributed 

 through the mesophyll. The xylem here again 

 has thicker walls than the phloem and is stained 

 pink. 



3. Epidermis and cuticle of the under surface 

 of the leaf. Note the specialized epidermal 

 cells called guard cells. They occur in pairs 

 with a pore or stoma (plural: stomata) be- 

 tween them. Gas exchange and water loss 

 occur through the stomata. The guard cells 

 are capable of swelling or shrinking, through 

 changes in osmotic pressure, depending on the 

 light and other conditions. When they are 

 turgid, the stomata are open; when they wilt, 

 the stomata close. This is an important regu- 

 latory mechanism in the leaf. 



Root anatomy 



Study the prepared slides of median longitu- 

 dinal sections of root tips of corn {Zea mays). 

 Identify the root cap, the meristematic zone, the 

 zone of cell elongation, and the root hairs. The 

 meristematic zone provides all the new cells for 

 the growth of the root. 



Also examine the slides of a mature root of the 

 dicot, buttercup {Ranunculus). Three layers 

 should be distinguished: the outer epidermis, 

 from which the root hairs arise; the cortex, made 

 up primarily of cortical parenchymal cells, which 

 may contain starch grains (stained violet); and 

 the stele, a cylindrical area enclosing the con- 

 ducting elements. Note that the xylem (thick- 

 ened walls, stained red), in the center, is in the 

 form of a star. The phloem cells are located be- 

 tween the arms of the xylem. 



