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. (Quvirandrd) fenes trails 

 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 n r _ ' . r FIG. 9 1. Perforated leaf of Apono- 



and flora or b lagellates, Rotifers, geton fenestrdiis, Hook. f.=ov- 

 Bacteria and Algae accumulate in ran . d Sftraiis Poir. Lace- 



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



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). 



