OF NAIAS GRAMINEA^ VAR. DELILEI. 71 



from Kasr-Dachl and Chargeh had leaves without bast- 

 nerves^ and 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 Cordofan, 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 'Botanische Zeitung^ for 1874, pages 641-644. 



'' These explanations would, however, seem to be some- 

 what contradictory, seeing that the English specimens are 

 remarkable for their great length of leaf, whereas the 

 leaves of N. graminea from Cairo and Damietta are very 

 short. But a minute examination 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 Enghsh specimens have longer leaves 

 without bast-nerves ; while the Egyptian specimens have 

 shorter leaves without bast-nerves. Thus, again, we find 

 the N. 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 stag- 

 nant waters of the overflowed Nile, as in these stagnant 



