SPERMATOPHYTA 



363 



having floral parts more or less spirally arranged, while the flowers of the 

 Metachlamydeae are all definitely cyclic. The Archichlamydeae are 

 generally regarded as the ancestral stock from which both the Metachla- 

 mydeae and the Monocotyledoneae have been derived. 



Vegetative Organs 



The sporophyte of angiosperms presents an enormous diversity in size 

 and habit, ranging from tiny herbs to tall trees 100 m. in height. Most 

 angiosperms grow erect upon the ground, but some are trailing, climbing. 



Fig. 309. Cross section of a leaf of lilac {Syringa vulgaris), X250. Beginning at the top, 

 the tissues are the upper epidermis, the palisade layer, the spongy tissue with numerous 

 intercellular spaces, the lower epidermis in which three stomata are seen, and, in the 

 center, a small vein. 



or epiphytic. Their stems are usually branched, but may be unbranched; 

 they are generally aerial, but may be subterranean. As in gymnosperms, 

 branching of the stem is lateral, never dichotomous. The leaves are 

 typically broad and thin, but display extreme variation in size, shape, 

 and other features. Their arrangement on the stem may be either spiral 

 or cyclic. They may be simple or divided into leaflets (compound), pet- 

 iolate or sessile, net-veined or parallel-veined, deciduous or evergreen. 

 In net-veined leaves the veins form an obvious reticulum and their ulti- 

 mate veinlets end freely to form an open system. In parallel-veined 

 leaves the larger veins run parallel to one another and, if connected by 

 cross veinlets, these form a closed system. 



Leaf Structure. In spite of their diversity in external features, the 

 leaves of angiosperms are rather uniform in structure. Leaves arise as 

 lateral outgrowths from the embryonic region of a stem tip and develop by 

 intercalary growth. A cross section of a typical mature leaf reveals the 

 following tissues (Fig. 309) : the epidermis, usually a single external layer 

 of colorless cells with cutinized outer walls and containing numerous 



