The Plant Kingdom - 621 



ANGIOSPERMS 



Fig. 31-29. Diagram showing an evolutionary trend in the Plant Kingdom. Progressively 

 the diploid (2n) sporophyte generation has become larger and more dominant; while 

 simultaneously the haploid (n) gametophyte generation has diminished, becoming more 

 subordinate and dependent. Among many algae the diploid generation is represented 

 solely by the zygote. (From The Plant World.) 



or vessels (Fig. 13-8), and tracheids (p. 244) 

 may also be present. Typically the leaves are 

 broad, flat, and very efficient as photosyn- 

 thesizing structures. 



Two important angiosperm subclasses 

 have been evolved. These are: Subclass 1, the 

 Monocotyledoneae, and Subclass 2, the Dicot- 

 yledoneae. Each of these groups displays a 

 number of distinctive features. 



The Monocotyledoneae include the grasses, 

 palms, lilies, irises, tulips, sedges, cattails, 

 bananas, orchids, cannas, etc. Among all of 

 this wide variety of plants: (1) the embryo 

 displays just one cotyledon; (2) the petals 

 and other floral parts occur in groups of 

 three (or a multiple of three); (3) the leaves 

 tend to be elongate, narrow, and nonlobu- 

 lated; (4) the leaf veins run parallel to each 

 other and to the margins of the leaf; (5) the 

 vascular bundles of the stem do not have a 

 ringlike arrangement but are scattered; and 

 (6) cambium is lacking in the adult stem. 



The Dicotyledoneae, on the other hand, 

 include: many trees and shrubs (oaks, maples, 

 elms, apples, viburnums, laurels, etc.); most 

 truck garden plants (spinach, broccoli, leg- 

 umes, tomatoes, potatoes, etc.); many flower 



garden plants (roses, phlox, geraniums, pop- 

 pies, sunflowers, etc.); many wild flowers 

 (dandelions, buttercups, violets, hepaticas, 

 etc.), and thousands of other plants, both 

 common and uncommon. All these plants 

 display: (1) tivo embryonic storage leaves 

 (cotyledons); (2) petals and other floral parts 

 occurring in groups of five (sometimes four); 

 (3) broad, frequently lobulated leaves; (4) a 

 branching network of veins in the leaves; 

 (5) a circular arrangement of vascular bun- 

 dles in the stem; and (6) a ring of cambium 

 in the stern. 



The basic nature of flowers and fruits, and 

 how these structures contributed to the evo- 

 lutionary success of the Angiospermae were 

 considered in Chapter 12. Paleontological 

 evidence as to the precise origin of the angio- 

 sperms is somewhat scanty. Probably this 

 great group also originated from the "seed 

 ferns" (p. 563). The first unmistakable fossils 

 of true flowering plants occur in Cretaceous 

 strata, formed about 125 million years ago. 

 These fossils show, however, that many of 

 the modern angiosperms (elms, maples, mag- 

 nolias, sycamores, poplars, etc.) had become 

 recognizable even in these early times. 



