vi] WINTER-BUDS OF GREATER DUCKWEED 75 



Such turions are of great importance throughout the family, 

 since the flowers are rare and relatively little seed is set. The 

 winter-fronds of Spirodela are smaller than the summer ones 

 and almost kidney-shaped. The air spaces in the tissues are 

 reduced, and the cells are packed with starch, with the result 

 that the fronds are heavier than water. The roots remain un- 

 developed. These winter-buds become detached from the 

 parent frond in the autumn and sink to the bottom of the water. 

 In the spring, a lateral frond begins to grow out; in so doing it 

 absorbs the starch from the parent, and on this account, and also 

 by development of air spaces, the whole body becomes lighter 

 and rises to the surface 1 . The present writer has found that the 

 rising of the winter-buds can be induced, as early as January, 

 as a result of a few days in a warm room, even in a dim light. 

 The time of year at which the turions begin to be formed is 

 variable, and depends on external conditions. It has been shown 

 by van Horen 2 that in shady places they develop very late or 

 even fail altogether, whereas they occur early in bright sun- 

 light, especially if the water is stagnant. Guppy 3 , who has made 

 a special study of the habits of the Lemnaceae, mentions that 

 on one occasion he found a large number of plants of Spirodela 

 polyrrhiza in a ditch, producing winter-buds, at the beginning 

 of July, to an extent he had never seen before or since; the 

 conditions were precisely those indicated by the previous ob- 

 server as being favourable to the early occurrence of this phase 

 namely almost stagnant water which was brilliantly insolated. 

 During the few weeks preceding the observation of the winter- 

 buds, Guppy records that the surface was frequently heated in 

 the day time to 80 Fahr. (nearly 27 C.). It is difficult to 

 understand why conditions so favourable for vegetative growth 

 should initiate turion formation, since in most water plants 

 their production is induced by a state of poor nutrition. Pos- 

 sibly the explanation may lie in the great size of the winter-bud 

 of the Lemnaceae in relation to the entire vegetative body of the 



1 Hegelmaier, F. (1868). 2 Horen, F. van (1869). 



^ Guppy, H. B. (1894*). 



