82 PISTIA [CH. 



The Lemnaceae are generally regarded as related to the 

 Aroids, so it may be well to conclude this chapter by a 

 further reference to Pistia Stratiotes, L. 1 , the River Lettuce of 



FIG. 53. Pistia Stratiotes, L. A, radial longitudinal section of leaf apex showing 



groove into which the water pores open and the space beneath them into which 



tracheids emerge. B, surface view of water pore. [Minden, M. von (1899).] 



the Tropics the member of the Araceae most nearly allied 

 to the Duckweeds. This plant has a floating rosette of leaves, 

 and multiplies by runners from which fresh rosettes arise. The 

 lower side of each sessile leaf bears a swelling, which may 

 reach the size of a pigeon's egg. This swelling consists of 

 spongy air-containing tissues, and serves as a float. The upper 



i On Pistia see Koch, K. (1852), Hofmeister, W. (1858), Engler, A. 

 (1877) and Arber A. 



