74 LEMNACEAE [CH. 



in his monograph of the Lemnaceae, treats them as stem organs 

 which are modified to perform the work of leaves. Engler 1 , on 

 the other hand, follows van Horen 2 in in- 

 terpreting the distal end of the frond as 

 foliar, while the proximal end is axial. Yet 

 a third view is that of Goebel 3 who expresses 

 the opinion that the leaf-like organs of the 

 Lemnaceae are actually leaves, pure and 

 simple. He explains the origin of the lateral 

 shoots of each generation from the base of 

 the preceding one, by assuming that the base FIG. 47. spirodeiapoiyr- 

 of each leaf has the power of functioning rhi * a > Schl f d - * nfl r - 



r o escence. st t and s/ 2 , 



as a growing point. Undoubtedly Engler's male flowers reduced 



i i i i i to stamens; c., female 



view which is based upon a comprehen- flower re duced to a 

 sive study of the Araceae, and a critical gynaeceum;s.,spathe; 



/ r ~. i i -r /., lateral shoot. [Hegel- 



exammation of Ptstta and the Lemnaceae maier, F. (1871).] 

 may be accepted as the best founded. The 

 present writer has recently carried Engler's comparison further, 

 and has shown that the buds in the case of Pistia arise in 

 minute pockets closely recalling those of the Duckweeds 4 . 



The three genera into which the family is divided Spiro- 

 dela, Lemna and Wolffia are all represented in Britain. 

 Spirodela polyrrhiza, Schleid. 5 , is the largest member of the 

 Lemnaceae; when it is growing vigorously its fronds attain to 

 about J- of an inch both in length and breadth. Several roots 

 with conspicuous root-caps hang from the underside of each 

 frond. They are somewhat heavier than water and their tips 

 are the heaviest part. It has been suggested that one of the 

 functions of these roots may be to ensure the equilibrium of the 

 plant 6 . Spirodela forms special shoots which outlast the winter. 



1 Engler, A. (1877). 2 Ho ren, F. van (1869). 



3 Goebel, K. (1891-1893). 4 Arber, A. (i9i9 4 ). 



5 For a description of the very rare flowers of this species see Hegel- 

 maier, F. (1871). 



6 Gasparini, quoted by Hegelmaier, F. (1868); Ludwig, F. in Kirch- 

 ner, O. von, Loew, E. and Schroter, C. (1908, etc.). 



