MONOCOTYLEDONS 197 



like points directed downwards. The significance of 

 this arrangement will be explained when we come to 

 deal with pollination devices, in Chapter X. The 

 specific name, maculatum, means " spotted," and it has 

 reference to the brown spots which are usually dis- 

 played by the stalked spear-shaped leaves. It is a 

 matter of observation that in some districts these spots 

 are never seen on the leaves. About 900 species of 

 Aroids have been distinguished; the great majority are 

 found in the tropics, but about eighty occur in temperate 

 climes. Some tropical species climb up trees, attach- 

 ing themselves to the stems by aerial roots ; they reach 

 light and air at the tops of the trees and frequently 

 send down long, hanging roots into the moist air of 

 the jungle; sometimes these pendant roots reach the 

 ground and obtain nourishment therefrom. 



The Duckweeds (Lemnacece) show affinities with the 

 Aroids, and are considered by some botanists to be de- 

 graded forms of specialized types. Whether they be 

 degraded or no they are eminently successful in the 

 struggle for existence, for they are represented in all 

 latitudes; yet there are only two genera embracing about 

 twenty species. The Lesser Duckweed, Lemna minor, 

 is practically ubiquitous in its distribution. All the 

 Duckweeds are floating plants of small size; they usually 

 propagate by budding or by hibernating bulbils; they 

 rarely produce seed. The flowers, when produced, 

 are extremely simple. They rise from a fissure in the 

 upper surface of the green scalelike leaf, or in its edge ; 

 they consist of a very small membranous spathe enclos- 

 ing a single carpel and one or two stamens. Four 

 species of the genus Lemna are found in Britain. The 



