vi] THE GIBBOUS DUCKWEED 77 



history, flat fronds are however produced and we owe to Guppy 1 

 the elucidation of the part played by the two types of shoot. He 

 observed one hot summer, when Lemna gibba flowered profusely 

 in July, that, during August, the gibbous plants gave rise to 

 numerous thin, flat fronds of a dark green hue. These were the 

 turions, and their appearance was accompanied by the death of 

 a large number of the gibbous mother-plants, a result which this 

 author attributes to exhaustion after flowering. Many of the 

 gibbous plants, however, survived and continued to bud off 

 winter-fronds except during the severest weather. Early in Feb- 

 ruary the budding recommenced, but the gibbous character was 

 not displayed until the weather became warmer. This author 

 thinks that for the development of the gibbosity the plants re- 

 quire an average daily maximum temperature at the surface of the 

 water, not much, if at all, under 70 Fahr. (21 C.). After cool 

 summers when Lemna gibba does not flower, no flat winter-buds 

 are formed, but the gibbous fronds survive until the next 

 spring. One of the reasons for the relative rarity of L. gibba, 

 as compared with L. minor, is probably that, as Guppy has 

 shown, it requires a higher temperature than that needed by 

 the Lesser Duckweed, both for initiation of budding in spring 

 and for flowering. Under suitable conditions, however, it shows 

 a wonderful vigour of vegetative growth. It has been recorded, 

 for instance, that an area of water of about half an acre, which 

 was edged on a certain date in June by a border of this plant 

 a few feet wide, nineteen days later was thickly covered with 

 the fronds over almost its entire surface 2 . 



The surface-living Duckweeds can survive for a time if 

 stranded on the mud by the lowering of the water in which they 

 grow, and in cultivation it has been found possible to establish 

 land forms which can fulfil the whole cycle of normal vegetative 

 development 3 . For instance, Lemna minor has been grown for 

 as long as twenty months on wet mud, where it throve and 

 budded at all seasons of the year. Two plants set apart in 



1 Guppy, H. B. (18942). 2 Hegelmaier, F. (1868). 



3 Guppy, H. B. (18942). 



