APPENDIX A 621 



Class Angiospermae 

 The Higher Seed Plants 



The Angiosperms form the third class of the subphylum Pteropsida, 

 or the second subdivision of the division Spermatophyta. These are the 

 flowering plants, in which the seeds develop within the ovary of a flower. 

 The group comprises about 133,000 species, which include herbs, shrubs, 

 vines, and trees. Most of the plants important to man as sources of food, 

 clothing, and industrial materials are angiosperms. The group is often 

 characterized as "the broad-leaved plants," by contrast with the conifers. 



Gymnosperms and angiosperms are similar in producing seeds, as opposed to the 

 three lower divisions. The angiosperms differ from the gymnosperms, however, 

 in the following respects: (1) the presence of vessels in the xylem; (2) the production 

 of flowers and fruit; (3) the formation of a pistil, to the apex of which pollen 

 adheres and through which the pollen tube must grow to reach the ovule; 1 (4) the 

 predominance of insect pollination (though wind pollination also occurs in several 

 large groups) ; and (5) the further reduction of the gametophyte to merely a few cells 

 existing but a few days. 



The angiosperms include some 300 families of plants, but these can be grouped 

 into two main subdivisions — monocotyledons and dicotyledons. The principal 

 distinctions between these two groups are as follows: 



1. The leaves of monocotyledons are generally parallel- veined and almost 

 always have smooth, even margins, whereas the leaves of dicotyledons are gen- 

 erally net-veined and are very often toothed, lobed, or divided. 



2. The flowers of the monocotyledons are generally built on a plan of three; 

 i.e., the number of flower parts of any one kind (petals, sepals, etc.) is three or 

 some multiple of three. In the dicotyledons the number of parts of each kind is 

 generally four or five or some multiple of four or five. 



3. In the stems of the monocotyledons the conducting tissue is in numerous 

 vascular bundles scattered through the stem but not arranged in a single ring; 

 in dicotyledonous stems the conducting or vascular tissue either is a hollow 

 cylinder surrounding the pith and increasing in width as the stem grows older 

 or is distributed in separate bundles arranged in a single circle. 



4. The embryos of the seeds of monocotyledons have only one seed leaf (coty- 

 ledon), while in the dicotyledons there are almost always two. 



The Monocotyledons (subclass Monocotyledoneae, commonly called monocots) 

 number about 27,000 species. Only four of the more important families can be 

 mentioned. 



The Gramineae (grass family) includes a number of species that are of pre- 

 eminent economic importance. In the tropics bamboo is one of the most valuable 

 of plants, being used for a great variety of purposes. In the temperate regions 



1 In the gymnosperms, the wind-borne pollen falls into the angles between the 

 cone scales and is thence drawn up through the neck of the ovule by a sticky secretion 

 first extruded from the ovule and then resorbed; the pollen tube has merely to pene- 

 trate the inner layer of ovule tissues and is very short. 



