DUCKWEED 195 



They float in water, and germinate at the surface in 

 the course of the following spring. 



Lemna minor is found in every quarter of the 

 globe, though it is wanting, or at least undiscovered, 

 in most parts of the tropics. 1 



Winter is naturally very destructive to the floating 

 fronds of Duckweed. A frost kills many, and sends 

 them to the bottom. During the milder intervals 

 fresh fronds are budded out, but they get smaller and 

 smaller as the light and warmth decline. These 

 winter-fronds, which are often so small as to escape 

 the notice of any but a close observer, are very hardy, 

 and survive a hard frost in considerable numbers, 

 serving, together with the seeds, to perpetuate the 

 race. In spring they emit larger fronds, which 

 multiply with great rapidity, and soon cover the water 

 with a green carpet. I believe that the rapid budding- 

 out of the new fronds is materially aided by their 

 tendency to form strings and chains, which spread 

 loosely and irregularly over the surface of the water. 

 Most floating objects, such as seeds of water-plants, 

 or bits of stick, attract one another at all points, and 

 gather into a dense mass. But Duckweed fronds 

 attract one another at certain points only. Hence 

 they cling together in strings and stars with unoccu- 

 pied spaces between. Some simple experiments, 

 which can be easily set up in any household, will 

 illustrate the principle on which the difference de- 

 pends. ' Take a small cork, and cut it into a number 

 of slices. Set these floating on water The discs of 

 cork attract one another, and are attracted to the 



1 Hegelmaier, Die Lemnaceen, p. 142. 



O 2 



