i 4 2 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 1 



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. 2 , 



while in A. (Ouvirandra) fenestralis 



the mature leaves are completely 



reticulate (Fig. 91). According to 



M lle Sergueeff 3 , 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 



r >r 



and flora of Flagellates, Rotifers, 



Bacteria and Algae accumulate in 

 their neighbourhood, without ap- 

 parently being responsible for their actual initiation ; probably 



1 Henslow, G. (1893), 



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



3 Sergueeff, M. (1907). 



FIG. 91. Perforated leaf of -4ono- 

 geton fenestralis, Hook. i.-Ouvi- 



