28 ON NAIAS GRAMINEA DEL., VAE. DELILEI MAGNUS. 



that another set had them ; that is, that the one set belong to the 

 var. Delilei, while the other agrees with the form which appears in 

 Oordofan, Djur, Algiers, &c. This would appear to be a clear 

 proof that the oases of the Libyan Desert have received their flora 

 from Egypt as well as from Central Africa. This agrees with the 

 results of the investigations which Ascherson furnished to the 

 1 Botanische Zeitung ' for 1874, pages 641 to 644. 



" These explanations would, however, seem to be somewhat 

 contradictory, seeing that the English specimens are remarkable 

 for their great length of leaf, whereas the leaves of IV. graminea 

 from Cairo and Damietta are very short. But a minute examina- 

 tion of form teaches us that we must not attach much importance 

 to the question of the length of leaves, which is influenced, as in 

 most water-plants, by the depth, current, bed, and temperature of 

 the water. Thus we find that the specimens collected by Professor 

 Ascherson in the Dachl Oasis, from the deeper pools (half a metre 

 deep), have long leaves as well as bast-nerves, and yet the English 

 specimens have longer leaves without bast-nerves ; while the 

 Egyptian specimens have shorter leaves without bast-nerves. 

 Thus, again, we find the Nl graminea Del., growing in the shallow 

 ditches of the rice-fields of the plains of Lombardy, has short 

 leaves with bast-nerves, whereas the Najas graminea from Celebes 

 has very long leaves with bast-nerves. In short, we see that 

 the length or shortness of the leaves has nothing whatever to do 

 with the formation of the variety, and nothing to do with the 

 histological formation of the leaf-tissue. 



" It is nevertheless possible that the var. Delilei, deprived of the 

 bast-nerves, has been developed in the quiet stagnant waters of the 

 overflowed Nile, as in these stagnant waters the mechanical cells 

 would become deprived of their functions. Thus we find Schwen- 

 dener, in his exhaustive work, ' The Mechanical Principle in the 

 Anatomical Construction of Monocotyledons,' Leipzig, 1874, page 

 122, remarking that Potamogeton fluitans in its customary habitat 

 of running water has a developed system of bark-bundles, whereas 

 the var. /3 stagnalis Koch is completely deprived of same. 



" The var. Delilei, found in the stagnant waters of the 

 overflowed Nile, is a most persistent and constant one, as during a 

 period of a hundred years it has been indubitably collected by 

 Delile, Schweinfurth, and Ehrenberg, in Lower Egypt. Its 

 unaltered appearance in England and in the oases shows its 

 constancy and total independence of habitats, whilst its formation 

 has probably been caused by the same." 



It now only remains to me to tender my acknowledgments to 

 Mr. Eidley, Mr. Arthur Bennett, Dr. Magnus, Professor Ascherson 

 Mr. Beeby, and to the Editor of this Journal, for help rendered' 

 The delay which has occurred in completing this paper has been 

 unavoidable ; it has had to take its turn in the intervals of a busv 

 life. J 



