THE MONOCOTYLEDONES 2009 



The Lemnaceae are small, floating, fresh-water plants with much 

 reduced dorsiventral " fronds " of lenticular shape and consisting entirely 

 of parenchyma. 



There are three genera, Lemna (Fig. 1945), Wolffia and Sptrodela, all 

 of which are represented in this country. The genus Lemna contains six 

 cosmopolitan species, four of which occur in Britain. L. polyrrhisa is the 

 largest, and sometimes produces fronds about i cm. in diameter. The 

 roots hang down from the lower surface and have large, conspicuous root- 

 caps which may function as balancing organs to maintain the fronds the 

 right way up. The flowers of the Lemnaceae are reduced to a minimum. In 

 Spirodela the flower is composed of a minute spathe enclosing two male 

 flowers, each represented by a single stamen, and one female flower in the 

 form of a one-chambered ovary with one or two ovules. They are probably 

 pollinated by small aquatic insects for they are markedly protandrous. 



The genus Woljfia (Fig. 1946) contains twelve species and has the 

 distinction of being the smallest of all flowering plants. The largest being 

 about 1-5 mm. in diameter, a dozen flowering specimens could be accom- 

 modated on a single frond of Lemna minor. Species of the Lemnaceae 



Fig. 1946. — Woljfia anhiza. Vertical section 

 of entire plant. On left, a new frond 

 arising. In central cavity, an anther (left) 

 and a carpel (right). (After Le Maoitt and 

 Decaisne.) 



flower but rarely, propagation being mostly effected by caducous branches 

 which arise at the posterior end of the shoot. In L. trtstilca, the fronds of 

 which are lanceolate, many branches remain attached, forming extensive 

 compound plants, or they may become detached giving rise to 

 new independent plants which start growth the following spring, while the 

 mother plant sinks to the bottom of the pond, 



Araceae 



The Araceae are a large family occurring most typically in the warmer 

 parts of the world but extending into temperate regions. They are very 

 variable in habit, some having erect or even climbing stems and some having 

 subterranean tubers or rhizomes, while Pistia is a floating water-plant. 



