268 HOUSE, GARDEN, AND FIELD 



leaf of the float-grass, no matter what is the state of the 

 weather, no matter how roughly the leaves have been 

 treated, is always dry on its upper surface, and always wet 

 on its under surface. The dryness of the upper surface is 

 due to the deep furrows between the ridges. Into these 

 the surface-film of the water cannot pass, 1 and the water 

 above the surface-film is accordingly held up and pre- 

 vented from entering. No accident to which the float - 

 grass is exposed can fill the furrows with water, or drench 

 the stomates which lie sunk in them. There is another 

 Glyceria, almost equally common in watery places ; in 

 this second species (Glyceria aquatica) the leaves never 

 float, and it is interesting to remark that they have no 

 ridges on their upper surface. 



It is probable that grass-leaves originally became ridged 

 on their upper surfaces to facilitate rolling up lengthwise 

 during seasons of drought, but float-grass has turned its 

 leaf -ridges to account as a means of preventing the wet- 

 ting of the stomate-bearing surface. A cross-section of the 

 leaf reveals a number of enclosed air-spaces, which, one 

 would think, must greatly increase the buoyancy of the 

 floating leaves ; however, in the second species of Glyceria 

 (G. aquatica), whose leaves do not float, the air-spaces are 

 much larger. They are not simple cavities, but are filled 

 with stellate cells. 2 



What is the use of the colourless scale which is found 

 inside the leaf-sheath, just where the blade becomes free ? 

 I have puzzled over this question for years without the 

 least success. Some people think that the scale hinders 

 water from making its way into the sheath. It is an 

 objection to any such explanation that the surface-film 



1 Object-lessons from Nature, Pt. II., pp. 135-6. 



2 This was pointed out to me by Mr. Norman Walker in sections of 

 G. fluitans. Mr. Lewton-Brain in Linn. Trans., 1904, says that the "low 

 ribs " of G. fluitans probably have no significance as an adaptive character. 

 I suspect that he has not seen sections through floating leaves, where the 

 ridges are as sharp and distinct as possible ; in aerial leaves of the same species 

 the ridges are much lower, especially in the neighbourhood of the midrib. 



