92 



A. H. Graves, 



(2) Structure of Vascular Bundles. 



The axial bundle contains two or three annular tracheae in its 

 young stage, which generally break down in the mature leaf to 

 form an open passageway; surrounding this portion are phloem 

 cells and parenchyma, difficult to distinguish on account of their 

 low degree of differentiation, and about the whole axial bundle is 

 a pretty definite endodermis, which Sauvageau has also noted. 

 The structure of the two lateral bundles is extremely simple, 

 consisting merely of a very few small conducting cells (PI. VII, 

 fig. 43, lb). 



d. Lacunae. 



On each side of the central bundle, separated from it by two or 

 three layers of parenchyma, are the lacunae (PL VII, fig. 43, la). 

 In mature leaves these are of the shape of flattened cylinders, ex- 

 tending longitudinally through the leaf, and divided into compart- 

 ments by transverse, sometimes oblique, perforated, one-layered 

 plates of roundish cells (PL VI, fig. 25 ; Text-figs. 14-15). The per- 

 forations are round intercellular spaces, occurring regularly at the 



JJ'igure 14.— Longitudinal section of 

 lacnna, showing cross sections of 

 diaphragms ; d z', diaphragm ; / a, la- 

 cuna. X 186. 



Pig. 15. — Sui'face view of one 

 of the diaplu'agms which sep- 

 arate the leaf lacunae into com- 

 partments. The small circular 

 outlines represent spaces be- 

 tween the cells, x 350. 



angles of the cells and affording communication for the air or gases 

 from one compartment to the next. 



e. Comparison with Other Potamogetonaceae. 



The anatomy of the leaf of Potamogeton pectinatus, a plant so 

 much like Ruppia in external appearance, is almost a duplicate of the 

 leaf structure of Ruppia (Schenck, 1886, p. 16 and figure 11 d). 

 The other species of Potamogeton show differences more or less 

 great, according to the shape of the leaf and the external conditions. 

 The leaves of Zannichellia palustris (Schenck, 1. c.) and Althenia 



