142 TYPES OF LEAF IN WATER PLANTS [ch. 



ledons, the most familiar case being that of various Batrachian 

 Ranunculi. Among Monocotyledons the submerged leaves are 

 nearly always entire ; the character- 

 istic venation of this group does not 

 lend itself readily to the formation 

 of a dissected leaf. As Henslow^ 

 has pointed out, dissection among 

 Dicotyledons is represented, in the 

 very few equivalent cases among 

 Monocotyledons, by fenestration, 

 which produces a similar result. He 

 adds the ingenious, but probably 

 untenable, suggestion that the fene- 

 stration of the aerial leaves of Tor- 

 nelia, Monstera^ etc., is a character 

 handed down to them from aquatic 

 ancestors. Among the Aponogetons 

 we meet with a slight and irregular 

 perforation of the leaves in A. 

 Bernerianus^ (Decne.) Hook, fil.^, 

 while in A. (Ouvirandra) jenestralis 

 the mature leaves are completely 

 reticulate (Fig. 91). According to 

 M''^ Sergueeff^, who has made a 

 detailed study of the subject, the 

 young leaves are imperforate, the 

 perforations arising at a later stage 

 by destruction of the tissues. When 

 the perforations are formed, a fauna 

 and flora of Flagellates, Rotifers, 

 Bacteria and Algae accumulate in 

 their neighbourhood, without ap- 

 parently being responsible for their actual initiation; probably 



iRenslow, G. (1893). 



2 Krause, K. and Engler, A. (1906). 



3 Sergueeff, M. (1907). 



Fig. 91. Perforated leaf of Apono- 

 geton fenestralis, Hook. i. Ouvi- 

 ^'randra fenestralis, Poir. Lace- 

 plant. [Sergueeff, M. (1907).] 



