596 CONSERVATIVE ORGANS. [LECT. X. 



I have already demonstrated to you the exist- 

 ence of large vacuities in the foliar parenchyma of 

 the leaves of aquatic plants. Mirbel regards these 

 as accidental productions, rents or defects in the 

 cellular texture; an opinion, however,, which is 

 instantly refuted on a minute examination of these 

 parts. If we place a portion of the petiole, or any 

 of the larger costse of an aquatic, this thin trans- 

 verse slice, for example, of the petiole of Trapa 

 natans (fig. 12, Plate 10), under the microscope, 

 we perceive that the vacuities have a symme- 

 trical arrangement around the centre (c.), which 

 is vascular and consequently more opaque than 

 the rest of the slice; and that some of the va- 

 cuities are open (a. a.), while others (b. b.) are 

 closed. The membrane which covers some of the 

 vacuities as they appear in the slice (fig. 12.) 

 before us, is a diaphragm, which, as it forms the 

 roof of one cavity, is also the floor of another ; and 

 it is owing to these diaphragms not being all on the 

 same plane that some of the vacuities appear de- 

 void of them, in the transverse slice of the petiole 

 of the leaf of any aquatic plant. The intimate 

 structure of these diaphragms is seen in a highly 

 magnified view of one of them, as represented at 

 fig. 13, Plate 10 ; in which b. shows that the dia- 

 phragm consists of regular hexagonal framework, 

 with the intervening membrane either perforated 

 or studded with small transparent, amylaceous gra- 



