224 Pla?it Biology 



The blade may be simple, as in the case of the sunflower, or compound 

 (divided into leaflets), as in the horse chestnut. The margin of the 

 blade may be smooth or irregular. Within the blade are veins which 

 consist of vascular bundles. Each vascular bundle contains extensions 

 of xylem and phloem cells of the stele of the stem. 



The internal structure of a leaf may be studied from a thin cross 

 section. The upper surface is protected by the upper epidermis, the cells 

 of which contain cutin to prevent rapid loss of water. The lower sur- 

 face is protected by the lower epidermis. In the lower epidermis, and 

 less frequently in the upper, are slitlike stomata (Fig. 18) (singular, 

 stoma) which lead into intercellular air spaces within the leaf. Each 

 stoma is surrounded by a pair of semicircular guard cells which control 

 the size of the stoma. The guard cells contain green chlorophyll, while 

 the epidermal cells do not. The interior of the leaf is composed of meso- 

 phyll tissue made up of chlorophyll-bearing cells. Just beneath the upper 

 epidermis, the mesophyll is known as palisade tissue, because the chloro- 

 phyll-bearing cells are column-shaped and are arranged side by side 

 perpendicular to the surface of the leaf. The mesophyll below the pali- 

 sade is known as spongy tissue in which the irregular-shaped cells sur- 

 round the intercellular air spaces. The veins of the leaf extend through 

 the spongy tissue. Each vein consists of a vascular bundle with its thick- 

 walled xylem cells toward the upper side and the thinner-walled phloem 

 toward the lower. The smaller veins contain only tracheids, while the 

 larger ones contain tracheids, sieve tubes, and companion cells. The 

 chlorophyll of the palisade and spongy tissues manufactures plant food 

 through the process of photosynthesis. 



IV. THE FLOWER 



A flower is really a series of whorls of modified leaves borne at the 

 end of a stem (Figs. 55, 57, 58, and 71). Flowers vary greatly In differ- 

 ent plants, but the more important structures and function may be ascer- 

 tained from the simple flower of the buttercup {Ranunculus sp.) (Fig. 

 70). The end of the stem which bears the floral leaves is called the re- 

 ceptacle. The outer whorl of floral leaves is called the calyx, which is 

 composed of five small, yellowish-green sepals. Within the calyx is the 

 more conspicuous corolla composed of five larger yellow petals. The 

 petals and sepals together constitute the perianth. Passing inward from 

 the corolla we find numerous filamentous stamens, each of which con- 



