CHAPTER XXX 

 THE ANGIOSPERMAE : MONOCOTYLEDONES 



The Monocotyledones are Angiospermae in which only one cotyledon is 

 produced by the embryo. The radicle usually emerges first from the seed 

 and is followed by the cotyledon which surrounds the plumule. In some 

 however the cotyledon remains within the seed and the plumule may then 

 be surrounded by a special plumular sheath. The details of the various 

 types of seed germination occurring in the Monocotyledon have already 

 been described (see Chapter XXVI) and we need not repeat them here. 



In the mature plant the leaves are characteristically parallel-veined and 

 the ovate or linear types of leaf are most common. Compound leaves 

 rarely occur. The main stems are often short, for the herbaceous type 

 predominates, and are commonly subterranean, the aerial shoots being 

 only lateral branches. Bulbs, corms or tubers commonly occur as storage 

 organs while fleshy or thin, wiry rhizomes are very frequently met with. 

 This fundamentally herbaceous type may however assume enormous pro- 

 portions and some herbaceous forms, such as the Banana, reach the size of 

 trees. In contrast to this we have the equally common grass type in which a 

 stiff, slender aerial stem is produced on which is borne a number of long 

 slender leaves. Many of these plants are annuals but others are perennial. 

 This type includes all the cereal crops so essential for human economy. 



Aquatic plants, both floating and submerged, are found in considerable 

 numbers while plants inhabiting marshes are equally common. 



Shrubs and climbing plants are far less common and are restricted to a 

 few families while the truly arborescent habit is restricted to the Palms and 

 a few genera of the Agavaceae, e.g.. Yucca. 



Anatomically the Monocotyledons show a marked contrast to the 

 Dicotyledons. The primary root rarely persists beyond the seedhng stage 

 and is soon replaced by a group of adventitious roots arising from the stem. 

 These roots are structurally simple and do not exhibit secondary thickening. 

 The stem structure is simple and consists of a large number of small, 

 closed, collateral vascular bundles scattered in a ground tissue. No true 

 secondary thickening similar to that in the Dicotyledons ever occurs, but 

 in a few genera a cambium may become differentiated in the outer part of the 

 stem from which additional vascular bundles are formed centripetally. Xo 

 solid mass of wood tissue is however produced. 



The leaves are usually simple, with a sheathing base, and are parallel- 

 veined. Cordate leaves with reticulate venation are characteristic ot a 

 number of genera in the families Dioscoreaceae, Araceae, Alismaceae and 

 Smilacaceae, especially in the tropics. The leaf may grow to a very large 



1965 



