2] THE VARIOUS GROUPS OF PLANTS 73 



and of many in the water, and comprising almost all agricultural 

 and horticultural crops as well as the majority of forestral products. 

 To one or other of these aspects the remainder of this book will 

 bear such abundant testimony that it would be superfluous to give 

 details here, though the reader may be referred especially to Chapters 

 XII-XVI for vegetational aspects and to Chapter IX for economic 

 ones. It is by the distribution and growth potentialities of angio- 

 spermic plants, more than any other group, that human migrations 

 have been affected in the past, and civilizations have been caused 

 to wax and wane. 



In view of their general importance it seems desirable here to 

 point out the two main groups (subclasses if the Angiosperms be 

 considered a class) into which the latter are usually divided (though 

 the individual criteria are not infallible). These are the Mono- 

 cotyledones (Monocotyledons), characterized by having a single 

 seed-leaf (cotyledon), and the more numerous Dicotyledones 

 (Dicotyledons), characterized by having two seed-leaves. In addi- 

 tion, the Monocotyledons usually have {a) narrow leaves with 

 parallel veins, and (b) the vascular bundles in the stem loosely 

 scattered and unable to extend ; also (c) rarely any woody develop- 

 ment, and (d) the flower-parts most often in whorls of three. The 

 Dicotyledons, on the other hand, usually have (a) broad foliage 

 leaves with net-like veins, and (b) the vascular bundles disposed in a 

 ring in the stem and commonly able to extend indefinitely ; also 



(c) often extensive woody development to form shrubs or trees, and 



(d) the flower-parts most commonly in fours or fives. Examples 

 of Monocotyledons are the Grasses, Sedges, Aroids, Orchids, Palms, 

 and Lilies ; of the Dicotyledons, most forest trees (other than 

 Conifers, Palms, etc.), members of the Pea family, and such crops 

 as Beets, Cabbages, Tomatoes, and Cucumbers, in addition to the 

 majority of broad-leafed herbs and shrubs. 



Further Consideration 



Many more details and illustrations of each of the main systematic 

 groups of plants may be found in almost any modern textbook of general 

 botany, such as R. D. Gibbs's Botany ; an Evolutionary Approach (Blakis- 

 ton, Philadelphia & Toronto, pp. xiii + 554, 1950), or R. C. McLean & 

 W. R. Ivimey-Cook's Textbook of Theoretical Botany, vol. I (Longmans, 

 London etc., pp. xv f 1069, 1 951)— or, for the predominant Angiosperms, 

 vol. II (ibid., pp. xiii ^- 1071-2201, 1956). It should, however, be 



