[73] 



CHAPTER VI 



THE LIFE-HISTORY OF THE LEMNACEAEi 



AND OF PISTIA 



EACH of the families with which we have been con- 

 cerned in the preceding chapters, has shown very great 

 variation in vegetative structure associated with the differing 

 degrees in which its members have adopted the aquatic habit. 

 In the Lemnaceae, which we propose now to discuss, we have, 

 on the other hand, a remarkably sophisticated and uniform 

 group of plants, all of which pass their life floating at or near 

 the surface of the water; the members of the family show, 

 throughout their structure, a high degree of similarity to one 

 another, and a marked difference from other aquatics. The 

 Duckweeds have a very wide range, and occur almost as 

 generally in the Tropics as in the northern countries where we 

 know them so well^. 



In the Lemnaceae the modification of the vegetative body 

 has been carried so far that the usual distinction between stem 

 and leaf is no longer obviously maintained. The Duckweeds 

 are not unique in this disregard of morphological categories 

 two other groups of water plants, the Utricularias and the 

 Podostemaceae, carry this infringement of botanical conven- 

 tions to an even more extreme point. 



The little green fronds of the Duckweeds produce similar 

 fronds of the second order, and also inflorescences of an ex- 

 tremely reduced type (Fig. 47, p. 74 and Fig. 50, p. 79) from 

 pockets occurring on either side in the basal region. The nature 

 of the fronds has been very variously interpreted. Hegelmaier ^, 



1 Hegelmaier, F. (1868) is still the classic monograph of this group. 

 See also Schleiden, M. J. (1839) and Hegelmaier, F. (1871) and (1885). 

 2Kurz, S. (1867). 

 3 Hegelmaier, F. (1868). For another view see Dutailly, G. (1878). 



