1966 A TEXTBOOK OF THEORETICAL BOTANY 



size. Some of the entire leaves of the Banana may be 8 to 10 ft. long, while 

 in the Palms the leaves may attain enormous size. They consist of a strong, 

 well-developed leaf-sheath, prolonged into a stout petiole and bearing a 

 huge, spreading, palmate or pinnate lamina. 



A theory was propounded by de Candolle and later elaborated by 

 Henslow, Arber and others, to the effect that the leaf in Monocotyledons is 

 only a phyllode, corresponding to the base and petiole, or in some cases only 

 to the base, of the leaf in Dicotyledons. On this view there is no true 

 lamina in Monocotyledons. (See Vol. I, p. 997.) 



The floral organs are usually arranged in five whorls with three members 

 in each whorl. In more specialized members one or more whorls may be 

 absent and the number of members reduced. A solitary bracteole is 

 characteristic and is situated either in a posterior or rarely in a lateral 

 position. 



It is probable that within the Monocotyledons we may trace two series, 

 the one starting with a simple generalized type of flower which gradually 

 becomes elaborated into a highly specialized structure, associated with a 

 very elaborate pollination mechanism. The second is a gradual reduction of 

 the floral parts culminating in the very reduced flower found in the 

 Lemnaceae. 



The seed is usually endospermic and sometimes has perisperm as well. 

 In the Orchidaceae, however, there is no endosperm and as the embryos 

 are extremely small the seedling relies on a symbiotic fungus to supply its 

 food requirements during the early stages of development. 



The classification of the Monocotyledons varies according to the views 

 of difl^erent authorities, but there is a greater degree of uniformity in the 

 methods here than we found in the Dicotyledons. There are several clear 

 and well-defined series, illustrating separate developmental lines, but all 

 showing affinities with one or more of the other groups. 



Speculation has naturally been aroused as to whether the Monocotyle- 

 dons or the Dicotyledons should be regarded as the more primitive. In the 

 light of recent work it is now generally accepted that both groups originated 

 from a common ancestral stock before Tertiary times, but there is little 

 indication among present-day forms of any family in either group from 

 which the opposite group might have arisen. The closest approximation 

 between the groups is shown by the Ranales and the Helobieae, but this 

 does not necessarily mean that the one has arisen from the other. 



HELOBIEAE (Alismales) 



The Helobieae are Monocotyledons in which the flowers are either 

 hermaphrodite or unisexual, regular and either naked or with a simple or 

 double perianth. There are an indefinite number of stamens occasionally 

 reduced to one. The carpels may also be indefinite or reduced to one. The 

 embryo is large and there is no endosperm. 



Most authors are agreed that the Helobieae are undoubtedly primitive, 



